[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

java/org/rocksdb/benchmark/DbBenchmark.java

@@ -534,6 +534,8 @@ public class DbBenchmark {
         (Integer)flags_.get(Flag.max_background_flushes));
     options.setCacheSize(
         (Long)flags_.get(Flag.cache_size));
+    options.setCacheNumShardBits(
+        (Integer)flags_.get(Flag.cache_numshardbits));
     options.setBlockSize(
         (Long)flags_.get(Flag.block_size));
     options.setMaxOpenFiles(
@@ -616,7 +618,6 @@ public class DbBenchmark {
     options.setCompressionLevel((Integer)flags_.get(Flag.compression_level));
     options.setMinLevelToCompress((Integer)flags_.get(Flag.min_level_to_compress));
     options.setHdfs((String)flags_.get(Flag.hdfs)); // env
-    options.setCacheNumshardbits((Integer)flags_.get(Flag.cache_numshardbits));
     options.setStatistics((Boolean)flags_.get(Flag.statistics));
     options.setUniversalSizeRatio(
         (Integer)flags_.get(Flag.universal_size_ratio));