kv/RocksDB: Add instrumentation to BinnedLRUCache

kv/RocksDB: Add instrumentation to BinnedLRUCache

1) perf counters
For each cache section in perf counters is created.
    "rocksdb-cache-O": {
        "capacity": 134217728,
        "usage": 134182832,
        "pinned": 0,
        "elems": 24502,
        "inserts": 25806978,
        "lookups": 150436987,
        "hits": 124629911,
        "misses": 25807076
    }
In default configuration there are 2 sections: "rocksdb-cache-O", "rocksdb-cache-default".

2) admin command
For each cache admin command "rocksdb show cache x" is added.
> ./bin/ceph tell osd.0 rocksdb show cache O
shard  capacity     usage    pinned     elems   inserts   lookups      hits    misses
    0  13631488  11076400         0      2099    136987    822679    685923    136756
    1  13631488  11549712         0      2043    133359    571500    438383    133117
    2  13631488  11060608         0      2232    135076    908468    773313    135155
    3  13631488  11166896         0      2269    134006    427070    293147    133923
    4  13631488  11117984         0      2297    133367    700242    567318    132924
    5  13631488  11306672         0      2155    137501   1130135    991810    138325
    6  13631488  11506512         0      2353    134515    662792    528514    134278
    7  13631488  11093856         0      2316    135348    718971    583421    135550
    8  13631488  11660624         0      2424    137363   1092043    954248    137795
    9  13631488  10962000         0      2561    131982    431702    300467    131235
   10  13631488  11379392         0      1916    134543    477118    342854    134264
   11  13631488  11294272         0      2555    134508    512393    378337    134056
   12  13631488  11277136         0      2079    137312   1131571    993692    137879
   13  13631488  10887776         0      2543    134001    567073    432903    134170
   14  13631488  10986528         0      2394    133288    584452    451018    133434
   15  13631488  11954464         0      2456    134615    708285    573374    134911

3) admin command
" ./bin/ceph tell osd.0 rocksdb reset cache O" clears "inserts",
"lookups", "hits" and "misses"

Signed-off-by: Adam Kupczyk <akupczyk@ibm.com>