[RocksDB] Expose stats to Perfrunner/CBMonitor

It would be useful to expose some RocksDB specific statistics to Perfrunner/CBMonitor for analysing the results of Perf Tests.
Following a meeting with Pavel Paulau, we have two options to achieve that:

1. Updating ns_server adding the new stats to what is currently available, e.g. http://cbmonitor.sc.couchbase.com:8080/ns_serverhera_510-1544_access_5139bucket-1 . Doing this way, we can skip the RockDB specific stats when not running under RocksDB using the Perfrunner filters (e.g., if all the measurements are zero, then we can skip the stat).
2. Adding a new Collector in Perfrunner to collect stats using directly the memcached stats API.

It seems to me that 1) is probably easier.

This is the list of stats that we would like to expose on CBMonitor:

ep_rocksdb_kMemTableTotal

ep_rocksdb_kMemTableUnFlushed

ep_rocksdb_kTableReadersTotal

ep_rocksdb_kCacheTotal

ep_rocksdb_default_kSizeAllMemTables

ep_rocksdb_seqno_kSizeAllMemTables

ep_rocksdb_block_cache_data_hit

ep_rocksdb_block_cache_data_miss

ep_rocksdb_block_cache_index_hit

ep_rocksdb_block_cache_index_miss

ep_rocksdb_block_cache_filter_hit

ep_rocksdb_block_cache_filter_miss

ep_rocksdb_default_kTotalSstFilesSize

ep_rocksdb_seqno_kTotalSstFilesSize