memtx: replace direct function calls to calls via pointers from vtab

Previously in memtx space direct memtx_tuple_new/memtx_tuple_delete
function calls were used. Also pointers to functions, used for alloc/free
memory for memtx tuples are stored in tuple_format_vtab. Replaced direct
memtx_tuple_new and memtx_tuple_delete function calls in memtx_space to
calls via pointers from vtab.
Part of #5419