Make creation timestamp consistent for all tables of a snapshot

This mostly affects listsnapshots command as well as removal of a snapshot when it is on TTL.

Currently, there is not the same timestamp for "created_at" field in listsnapshots output so what might happen when TTL deletion kicks in is that only part of tables might be removed because other tables do not have to make it into that TTL threshold but they will be removed on the next SnapshotManager's removal round.