Closes #1599

Don't access uninitialized memory in ev_io_closing().

anfds array is initialized with zeroes, when it's reallocated,
but anfdmax is array size, not array last element, so
anfds[anfdmax] is garbage.

When we're "closing" an fd which has never been added
to anfds (ev_io_start()) wasn't called for it), we
must check that it's within array size boundaries first.

Since range_id_max is not updated, indexes created after recovery might
get the same ids as recovered ones. If this happens, the next recovery
will crash, because there will be two or more identical ids in the range
index file.

Prepare for #1599

vy_range_redistribute() doesn't work correctly if ranges are not sorted
in ascending order.

vy_task_{dump,compact}_{execute,complete} serve as trivial wrappers for
the functions doing real work. Let's fold them to make the code more
straightforward. No functional changes intended.

While we touch this code, let's also get rid of all the pointless
unlikely's - after all the functions are not called from hot paths so
there's no point in cluttering the code.

This is pure refactoring, isn't supposed to introduce any functional
changes.

It is only set and never checked.

vy_range_lock() is used in order to forbid scheduling two dump or
compact operation on the same range, so there's no point in locking and
then unlocking new ranges on compact completion, because it is run from
the same thread as the scheduler. This is a legacy of the past, when
vy_range_compact_commit() was called from a worker thread. Zap it.

It's not good to mess them all together in one struct. Let's use a
union.

Take into account all runs and in-memory index.

Follow up bfe23238

Add a test case for automatic cursor close on transaction
commit/rollback.
Fix a coding bug this test exposed.
Update comments.

Add a test case for gh-1606 "visibility of changes made
by a transaction in range queries".

These ara basic tests for transaction write set
iterators implemented by @a_lyapunov

As there is ->complete available now, we can release quota from there.

We always do after successful vy_run_write(), so let's move the code
there.

vy_dump_commit() can only be called after successful vy_dump_begin(),
which always creates a new run, so the check if (run == NULL) is
useless.

It doesn't work.

This reverts commit 7bc88b06.

In case of WAL write error, the transaction was rolled back
after successful vy_prepare(). In this case, rollback still tried
to remove the tx from the conflict manager. Double deletion lead
to crash.

Add a test case and do not remove tx twice.

Fixes gh-1681 "vinyl: crash in vy_rollback on ER_WAL_IO"

Use lsn instead of vlsn in scheduler.

Fix regression introduced by f991838a

Split pages properly.

Store offset of the end of msgpack data in the last offset table entry.

* Remove useless ./vinyl/lock and test/vinyl/suite.lua
* Fix vinyl/info.test.lua

Follow up ca00c503 and
          24a5e45b

Fixes #1743
Fixes #1749

All manipulations on range tree are now done from the tx thread, while
worker threads only write runs to disk, so the lock is not needed any
longer.

Call 'begin' from task->execute and 'commit' from task->complete.
'begin' writes runs to the disk, while 'commit' manipulates the range
tree, renames range files and updates range index. After this change we
don't need index->lock anymore - the next patch gets rid of it.

This patch makes vy_task look like a real mess - we probably should keep
arguments for different tasks as structs in a union. Also, the wrappers
around vy_dump_{begin,commit} as well as vy_range_compact_{begin,commit}
doesn't look very helpful. But let's leave mere cleanups and refactoring
up to the following patches.

Closes #1685

Call 'begin' from task->execute and 'commit' from task->complete.
'begin' writes runs to the disk, while 'commit' manipulates the range
tree, renames range files and updates range index. This is a step
towards removal of index->lock.

We need one more operation apart from 'execute'. It is 'complete', which
may be used to finish the task from the scheduler context and perhaps
schedule another task. So let's switch vy_task to vtable. Note,
'complete' is only called if 'execute' finished successfully.

It doesn't work if there are more than 2 sources. The fix is
self-obvious.

Closes #1725

Fixes #1695

 * Internally, libev maintains a list of watchers for each monitored fd.
   Add ev_io_closing(fd), does ev_feed_event() for each watcher affected.

 * Add coio_close()  [Public API]
   Wakes all fibers blocked in coio_wait() AND closes the fd.
   Prerequisite for #886 Non-blocking popen.

 * Lua socket switched to coio_close(); stop tracking
   waiters explicitly.

 * socket ::= { _gc_socket = <ffi:struct gc_socket> }, _gc_socket
   was previously known as socket. Gc_socket stores the fd in
   immutable field.

 * Overall socket module cleanup.