memtx: introduce interfaces for MemtxAllocator

That includes:
 - foreach_memtx_allocator() - for each basic allocator (Small and Sys
   so far) we declare corresponding MemtxAllocator. So to iterate over
   all allocators and invoke same function we are going to use this
   helper;
 - memtx_allocators_init() - invoke ::create() of each Allocator and
   corresponding MemtxAllocator;
 - memtx_allocators_destroy() - invoke ::destroy() of each Allocator
   and corresponding MemtxAllocator;
 - memtx_allocators_set_mode() - set given delayed free mode for each
   MemtxAllocator.

Follow-up #5419