Post-merge fixes: revert changes to libev configuration files.

Conflicts:
	mod/box/box.m
	test/box/admin.result

Conflicts:
	mod/box/box_lua.m
	test/box/lua.result
	test/box/lua.test
	test/box_big/sql.result

https://bugs.launchpad.net/tarantool/+bug/955226
Remove trailing spaces.

https://bugs.launchpad.net/tarantool/+bug/977026

Avoid race condition while wal writer is stopping but the wal writer thread
didn't have time to start working.

Only flush stdio buffer once when done processing
a bulk of requests. This avoids an unnessary syscall (write())
per request.

This reverts commit 8dc70aac.

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.