[Java] Add Java support for cache sharding.

Summary:
Add setCacheNumShardBits() and cacheNumShardBits() to Options.  This allows
developers to control the number of shards for the block cache.

Test Plan:
make rocksdbjava
cd java
make db_bench
./jdb_bench.sh --cache_size=1048576 --cache_numshardbits=6

Reviewers: sdong, ljin, ankgup87

Reviewed By: ankgup87

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19347