[2pt] Add new 'memtx_allocator' option to box.cfg{}

[TarantoolBot]

Add a new option to box.cfg{}

Product: Tarantool
Since: 2.10.0-beta1
Root document: https://www.tarantool.io/en/doc/latest/reference/configuration/#configuring-the-storage
SME: ?
Peer reviewer: @

Details

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.
Requested by @EvgenyMekhanik in tarantool/tarantool@1847ae0.