memtx: fix tuple garbage collection not resumed after snapshot

This is a degradation introduced by commit 26f7056f ("Introduce
internal database read view API"): tuple garbage collection is never
resumed after a snapshot. This happens, because we don't add engine read
views to the read view engine list on read view construction. As a
result, read_view_close() never closes the memtx engine read view.

NO_DOC=bug fix
NO_CHANGELOG=unreleased