memtx: enter delayed free mode once when snapshot starts

Currently, we call memtx_enter_delayed_free_mode() per each index,
in index::create_snapshot_iterator(), but there's actually no need to
bump the snapshot_version more than once per snapshot. Let's move it
to the place where we start checkpoint/join and drop memtx wrappers
around memtx_allocators functions.

This will simplify reworking the memtx read view API, see #7364.

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring