Conflicts:
	test/box/lua.result

(https://bugs.launchpad.net/tarantool/+bug/972345)

Make sure we do not try to destroy uninitialized
mutexes in the snapshot thread.

Implement FIFO policy for the wal writer queue.

This patch introduces wal_mode configuration variable,
which can take either "fsync" or "fsync_delay" values.

In "fsync" mode, we open the write ahead log in O_SYNC
mode, and thus sync every write to disk.

Th fsync_delay mode is the backward-compatible mode:
in this mode, we only do fsync once every fsync_delay
seconds. If fsync_delay is 0, there is no fsync.

Otherwise, this patch is only adding auxiliary headers,
object files, etc.

Conflicts:
	test/box/lua.result

Check grammar in comments. Fix an incorrect
assert. Add a test which triggers the assert.
Add a few more tests for combinations of insert, delete
and set.

review fixes:
 - update_op_cmp was improved;
 - update_field_init function was refactored;
 - field struct member 'old_end' was renamed to 'tail'.

Code review minor fixes.

Code review minor fixes.

New operation '!' (insert) to box.update Lua fuction was added.

Conflicts:
	test/connector_c/update.c
	test/connector_c/update.result

connector/c update.test result was updated.

Review fixes:
  - Minor comments fixes;
  - INSERT operation behavior was changed: now INSERT operation was allowed
    under field next to last.
  - UPDATE connector/c tests was updated.

Actually, make the feature work.
Add syntax to Lua to be able to delete fields.
Fix a bug when we delete all fields in a tuple and happily
crash thereafter.

- INSERT AFTER operation was deleted;
- INSERT BEFORE was renamed to INSERT;
- Tarantool connector/c was updated;
- Tests was updated.

minor stylistic changes