Shared block cache in RocksJava

Shared block cache in RocksJava

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