memtx: use stailq instead of lifo for tuple garbage collection list

To immediately delete tuples that were allocated before a read view was
created, but are not visible from the read view (see #7185), we will
need to store them in per-read view garbage collection lists. We will
also need to merge those lists. With lifo, merging is impossible. Let's
switch to stailq, which allows this operation (see stailq_concat).

Note, since the min value of box.cfg.slab_alloc_granularity is 4, we
align memtx_tuple by 4 bytes.

Needed for #7185

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring