A fix and a test case for gh-372

Fixes gh-372: assertion failure with a function
which inserts increasingly large tuples.

Make sure oom.test cleans up after itself.
Fix a slab leak when recycling a factored pool in small object
allocator.
Fix and optimize bitset index for case when key is empty

This patch fixes out of bounds memory access in bit_iterator_init
for size == 0. The patch also optimizes bitset/index library to skip
useless iterations over empty keys.

Closes #425
Merge branch 'stable'
Merge branch 'master' of https://github.com/tarantool/tarantool