[myrocks] Add rocksdb_block_cache_numshardbits for issue 1336 (facebook#1339)

Summary:
This fixes facebook#1336

This adds the my.cnf options: rocksdb_block_cache_numshardbits

This option can be set so that RocksDB to fix the number of block cache shards.

The default value is -1 to match existing behavior. When -1 RocksDB code will determine the number of block cache shards as min(6, rocksdb_block_cache_size / min_shard_size) and today min_shard_size is 512K for LRU and 32M for Hyper.

The math above frequently results in a block cache with too many small shards when rocksdb_block_cache_size is not too big (a few GB is not too big) and there will be perf problems that are hard to debug in such a case.

Pull Request resolved: facebook#1339

Differential Revision: D47635762

Author: mdcallag

mysql-test/r/mysqld--help-notwin.result

@@ -1323,6 +1323,8 @@ The following options may be given as the first argument:
  Deleting rows by primary key lookup, without reading rows
  (Blind Deletes). Blind delete is disabled if the table
  has secondary key
+ --rocksdb-block-cache-numshardbits=# 
+ Block cache numshardbits for RocksDB
  --rocksdb-block-cache-size=# 
  block_cache size for RocksDB
  --rocksdb-block-restart-interval=# 
@@ -2526,6 +2528,7 @@ rocksdb-allow-mmap-reads FALSE
 rocksdb-allow-mmap-writes FALSE
 rocksdb-allow-to-start-after-corruption FALSE
 rocksdb-blind-delete-primary-key FALSE
+rocksdb-block-cache-numshardbits -1
 rocksdb-block-cache-size 536870912
 rocksdb-block-restart-interval 16
 rocksdb-block-size 4096