* one step closer to libxrow - a standalone library to parse
  Tarantool snapshot, xlog, data and index formats
* one step towards killing request.h

Clean up #include headers.

TRASH() is already performed by mempool_free()

Closes #1709

Closes #1875

* Add missing pthread_key_delete()
* Add tt_pthread_XXX() wrappers
* Add comments to vy_env members

Fixes #1974

Fixes #1973

Currently, it doesn't look like a big win, but it will come in handy
soon.

In order to restore meta from memtx, we will need to be able to create a
range w/o an in-memory index.

Factor out pieces of code manipulating range->shadow and ->compact_list
to separate functions so that they could be reused on recovery vinyl
metadata from memtx.

Currently, we create runs in vy_range_write_run(), which is called from
a worker thread on dump/compaction. In order to move vinyl metadata to
memtx, we'll need to allocate an id upon run creation, which involves
modification of the memtx table, which can only be done from the tx
thread. So let's create the run for dump/compaction in advance: when
preparing for dump/compaction.

* separate MemtxSpace class into own files (declaration and definition)
* cleanup polluted includes in memtx_engine.cc

Use `gdb -batch -ex "generate-core-file" -p $PID` or `kill -SIGABRT $PID`
instead.

Closes #1886

Group encode/decode functions together.
No semantic changes.

Move stmt-related functions to the appropriate place.
No semantic changes.

* Fix dangling pointers in vy_page_read_task
* Remove fd and rc from vy_page_read_task
* Improve error handling

Fixes #1954

xlog tx iterator extracted from xlog cursor iterator. tx iterator may be
created from a memory chunk and now don't depend on xlog file.
Vinyl will use a task for page reading in online mode.

* Add coio_task_create() and coio_task_destroy()
* Rename coio_task() to coio_task_post()
* Fix the core invariant of coio_task_post():
  - On timeout or when fiber was cancelled set diag, return -1 and
    guarantee that on_timeout will be called somewhen.
  - Otherwise don't touch diag, don't call on_timeout callback,
    return 0. Please check task->base.result and task->diag
    to get the original return code and diag from the callback.
* Change the return value of coio_task_post() to "int".
* Add diag to coio_getaddrinfo() and fix a possible bug in replication;
  ignore uncoventional getaddrinfo(3) error codes for now.
* Fix buggy box.snapshot() tests.

Needed for #1954

Follow up the previous patch.

* Replace vy_index, vy_range and vy_run with more specific
  function arguments
* Add comments and fix code style
* Rename run_no to run_id everywhere

No semantic changes.

We will write all vinyl metadata in xlog file format. Each run has its
own xlog with one transaction. First xrow should contain an run creation
request, all others - page creation requests.

+ Remove support non-blocking I/O from fio_write().
  fio.h is for files only, please use sio.h for sockets.

Fix compilation warnings.

Follow up #1935 "minilua.o: relocation R_X86_64_32S against `.rodata'".

Fixes Debian Bug#846773: "ld: Nonrepresentable section on output".

Add test cases for transaction control statements
invoked in a trigger.

ABI change: box_txn_rollback() now may fail with error,
if called from a nested stateent.

Touch txn a little bit to support multistatement transactions in
autocommit mode.

This patch allow to perform DML requests from space:on_replace()
triggers. Trigerred requests are processed as a part of the original
transaction and entire transaction is rolled back in case of error.
Please note that there is still no support for nested transactions,
therefore a call to box.begin() from on_replace() trigger is not
allowed.

Recursive invocation is not forbidden, but limited to the maximal
depth of 4 to avoid stack overflow.

Proudly fixes #587

Closes #1378

Fixes #1962