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'