Summary:
Changes to support sharing block cache using the Java API.
Previously DB instances could share the block cache only when the same Options instance is passed to all the DB instances. But now, with this change, it is possible to explicitly create a cache and pass it to multiple options instances, to share the block cache.
Implementing this for [Rocksandra](https://github.com/instagram/cassandra/tree/rocks_3.0), but this feature has been requested by many java api users over the years.
Closes https://github.com/facebook/rocksdb/pull/3623
Differential Revision: D7305794
Pulled By: sagar0
fbshipit-source-id: 03e4e8ed7aeee6f88bada4a8365d4279ede2ad71
d5585bb605