memtx: change small allocator behavior

Previously, in small allocator, memory pools
were allocated at the request, which in the case
of a small slab_alloc_factor led to use
pools with incorrect sizes. This patch changed
small allocator behavior, now we allocate pools
on the stage of allocator creation. Also we use
special function to find appropriate pool, which
is faster, then previous version with rbtree.
This change fixes #5216.

Also moved a check, that the slab_alloc_factor is in
the range (1.0, 2.0] from small allocator to memtx_engine.
If factor is not in range change it to 1.0001 or 2.0 respectively

Closes #5216