Add ttl for cassandra snapshots

Currently Cassandra can create snapshots of SSTables containing all data of concrete Column Family (Analogue of the table into RDBMS).Snapshot is a copy of original data, represented by the hard link.They can be created before each compaction or before keyspace truncation.User also can manually create snapshot via nodetool snapshot command. After snapshot creation it is not managed by Cassandra.The goal of the task is to make it possible to set TTL to snapshot and automatically delete it after it’s expiration.