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