Summary:
Major changes in this PR:
* Implement CassandraCompactionFilter to remove expired columns and rows (if all column expired)
* Move cassandra related code from utilities/merge_operators/cassandra to utilities/cassandra/*
* Switch to use shared_ptr<> from uniqu_ptr for Column membership management in RowValue. Since columns do have multiple owners in Merge and GC process, use shared_ptr helps make RowValue immutable.
* Rename cassandra_merge_test to cassandra_functional_test and add two TTL compaction related tests there.
Closes https://github.com/facebook/rocksdb/pull/2588

Differential Revision: D5430010

Pulled By: wpc

fbshipit-source-id: 9566c21e06de17491d486a68c70f52d501f27687