Add two more options: disable block cache and make table cache shard number configuable

Summary:

as subject

Test Plan:

run db_bench and db_test

Reviewers: dhruba

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D6111

include/leveldb/options.h

@@ -271,6 +271,14 @@ struct Options {
   // exceeds rate_limit. This is ignored when <= 1.0.
   double rate_limit;
 
+  // Disable block cache. If this is set to false,
+  // then no block cache should be used, and the block_cache should
+  // point to a NULL object.
+  bool no_block_cache;
+
+  // Number of shards used for table cache.
+  int table_cache_numshardbits;
+
   // Create an Options object with default values for all fields.
   Options();