Creates snapshot properly by SIGUSR1

https://bugs.launchpad.net/tarantool/+bug/1162797

by SIGUSR1 tarantool
- didn't touch box.info.snapshot_pid
- didn't waitpid for snapshot process
socket-concurrent: implement exclusive per readers and writers socket access.
(https://bugs.launchpad.net/tarantool/+bug/1160869)
Creates snapshot properly by SIGUSR1

https://bugs.launchpad.net/tarantool/+bug/1162797

by SIGUSR1 tarantool
- didn't touch box.info.snapshot_pid
- didn't waitpid for snapshot process

Conflicts:

	include/fiber.h
https://bugs.launchpad.net/tarantool/+bug/1162797 review

Remove added logging, a message with 'saving snapshot %filename'
is already printed to the log by the child. It contains
snapshot pid. Snapshot fid is not as useful once the bug
is fixed.
Merge branch 'stable'

Conflicts:
	include/fiber.h
socket-concurrent: review fixes.
https://bugs.launchpad.net/tarantool/+bug/1160869, code review

Review comments.
Merge branch 'socket-concurrent'
Fix bug https://bugs.launchpad.net/tarantool/+bug/1163858

Replication relay was created (forked) after initialization
of slab allocator, and, consequently, had a very large address
space. The memory wasn't used really.

Initialize slab allocator after creation of a replication
relay process.

No test case since we don't have automation to detect that sort
of problems.
Merge branch 'stable'
A merge fix (https://bugs.launchpad.net/tarantool/+bug/1163858)
Merge branch 'master' of github.com:mailru/tarantool into snapshot_pid

Conflicts:
	include/fiber.h
	src/tarantool.m
Merge branch 'snapshot_pid' into remote
base functional is ready