This patch adds ability to reload C procedures on the fly without
downtime. To achive that, Tarantool loads a new copy of shared
library and starts routing all new request to the new version.
The previous version remains active until all started calls
are finished. All shared libraries are loaded with RTLD_LOCAL,
therefore two or more copies can co-exist without any problems.
From now box loads all external modules via an unique symlink to
avoid caching inside dlopen().

If one of some module function is reloaded then all other functions
from this module will be reloaded.

Reviewed and heavily patched by Roman Tsisyk.

Closes #910

rbtree_next() has log(N) complexity to advance to the right subtree.
The new iterators saves the iteration path during descent.

Closes #1668

panic() from non-main thread calls exit(), which fires
at_exit triggers, which try to destroy TX data.

Closes #2686

Memtx checkpointing subsystem uses createReadViewForIterator and
destroyReadViewForIterator methods of Index. So these methods
are used only for ALL iterators before actual iteration starts.

 * no other cases of these methods using are covered by tests,
for example EQ iteration or using in the middle of iteration.

 * creation of general read view iterator is an overkill for
such a simple case as ALL iterator.

 * memtx read view iterators are incompatible with lazy lookup
and restoration patterns that are necessary for implementing
stable iterators.

Introduce new createSnapshotIterator method that would create
a special iterator for checkpointing and remove misleading
createReadViewForIterator and destroyReadViewForIterator methods.

Make memtx tree iterator lookup lazy - in the first 'next' call.
It makes the iterator similar to vinyl in case when a new tuple
is inserted between a moment of iterator creation and a moment
of first tuple retrieval. Make the inserted tuple visible to
the iterator.

Store last returned tuple in the iterator and if the tree is
changed between iterations - search for the stored tuple and
continue iteration from the found position. This will make the
iteration stable and every next iteration will return proper
(next or previous) tuple.

Fix #1796

Originally bps_tree had only lower/upper_bound methods for
retreaving iterators by given key.

Sometimes it is necessary to find an iterator position by a value
that is (can be) stored in a tree.

For example memtx tree index holds a bps_tree with
(struct tuple *) values and (const char *) key.
lower/upper_bound methods used (const char *) as an argument,
but for stable iterators we need to find a position in the tree
by (struct tuple *) value.

Implement bps_tree methods that will allow to find an iterator
by a value (element) in the tree.

Needed for #1796

Move memtx_tree_compare_key definition to header file to allow
compiler to inline it.

Get rid of memtx_tree_comprare wrapper, use tuple_compare instead.

Comment the code.

xlog is used from Vinyl and Lua. Convert this library to C just for
sanity.

Closes #2335

* Move XlogError to error.cc
* Move XlogGapError to recovery.cc
* Replace tnt_error() with diag_set()
* Remove scoped_guard from xdir_scan()

A part of #2335

Update test results (changes not committed in scope of gh-2663).

Add a configuration option to configure the number of EIO worker
threads. Default value is 4.

Fixes #2663

Follow up the previous commit

See #2678

Fix regression after 9b811605

Closes #2678

Using %I is incorrect, because it is a result of unescaping, but
escaping never occured.
Even if we would use escaping inside tarantoolctl, it would be
inconvenient to use systemctl to operate instances.

Neeed for dynamic configuration.

+ Rename vy_update_options() to vy_set_timeout().

In context of #2667

Fix unhandled error in applier.cc.

The test spawns a number of test threads each of which randomly connect
to, disconnect from, and send messages to its neighbors.

cbus_endpoint->n_pipes is incremented from a thread connecting to the
endpoint (see cpipe_create()) while it is decremented from the thread
hosting the endpoint (see cbus_endpoint_poison_f()). While it is
protected with cbus->mutex in the former case, it is decremented
locklessly, which can result in a race leading to n_pipes never reaching
0 and hence cbus_endpoint_destroy() never returning. Fix this by taking
cbus->mutex for decrementing the variable.

Now memtx_tree sorts tuple by key_def and (if tuples are equal)
by pointer to tuple. That makes the  order of selection
unpredictable and mutable.

Order secondary non-unique tree index
first by secondary index key_def and then by primary key_def.

Remove is_unique check from memtx tree comparator since the
internal order is always unique.

Fix #2476

Key definitions are used in engines, do not pull it out from under
engine feet when performing an alter.

Since the secondary indexes order will be able to depend on
the order in the primary index, we need to rebuild them if the
primary index was changed.

Only non-unique tree indexes depend on the primary index, so rebuild
only non-unique tree indexes.

Add a test.

Needed for #2476

In the following patch I will need to alter struct alter_space
in AlterSpaceOp constructor. Pass alter_space in.

auth.test.lua was not converted to the new box.cfg{} options
and was printing warnings to stdout, which would confuse the
built-in tap detector in the harness.

Clean up everything after the test.

Fixes #2673

VinylIndex may be freed before vinyl iterator has been closed. Vinyl
iterator uses only an `env` item from corresponding VinylIndex, so store
environment pointer for iterators for direct use.

See #2673

vy_index may be deleted when cursor still exists, so reference
vy_index for cursor lifetime.

See #2673

Implement vy_cache_iterator_step, add comments.

Cache_iterator_next_key returns the same tuple as on previous step
if the cache version was changed.

Closes #2661

Mixing C-style error handling and C++ exceptions can lead to
serious problems.

In context of #910

Needed for #910

Get rid of unprotected usage of Lua API.

Needed for #910

Needed for #910

Optimize error_log(diag_last_error(diag_get())) pattern.

Create memtx format format in memtx engine, and vinyl format in vinyl
engine.
This is a pre-requisite patch for gh-2476 (predictable memtx tree
iteration).

The space may have a state, we should commit alter of the new space object,
not old one.

We can always use disk_format, in case of a single key space
it is identical to mem_format.

Add xlog_cursor crc, meta parsnig and reading error injections.
Add vy run data reading injection, forward injection from vy run data
reading to xlog.
See #1610