memtx: implement api for memory allocator selection

Add new 'memtx_allocator' option to box.cfg{} which allows to
select the appropriate allocator for memtx tuples if necessary.
Possible values are "system" for malloc allocator and
"small" for default small allocator.

Closes #5419

@TarantoolBot document
Title: Add new 'memtx_allocator' option to box.cfg{}
Add new 'memtx_allocator' option to box.cfg{} which allows to
select the appropriate allocator for memtx tuples if necessary.
Possible values are "system" for malloc allocator and
"small" for default small allocator.