Stats RPC to print out rocksdb memory stats

[wezrule]

This extends the stats RPC with another type called rocksdb

Example output is:

{
    "cur-size-all-mem-tables": "84208080",
    "size-all-mem-tables": "434238032",
    "estimate-table-readers-mem": "129269335",
    "estimate-live-data-size": "16350750564",
    "compaction-pending": "0",
    "estimate-num-keys": "70453544",
    "estimate-pending-compaction-bytes": "0",
    "total-sst-files-size": "18014738665",
    "block-cache-capacity": "260046848",
    "block-cache-usage": "132867094"
}

[guilhermelawless]

Example response

{
    "block-cache-capacity": "1140850688",
    "block-cache-usage": "884216512",
    "compaction-pending": "0",
    "cur-size-all-mem-tables": "52192",
    "estimate-live-data-size": "14672331",
    "estimate-num-keys": "157449",
    "estimate-pending-compaction-bytes": "0",
    "estimate-table-readers-mem": "300488",
    "size-all-mem-tables": "52192",
    "total-sst-files-size": "50024942"
}