Introduce new CRPCStats class to save key RPC statistics

[Jouzo]

What kind of PR is this?:

/kind feature

What this PR does / why we need it:

Save RPC latency and payload size on each call.

Introduces 2 RPCs:

• getrpcstats
• listrpcsstats

getrpcstats :

defi-cli -regtest getrpcstats getblockcount
{
  "name": "getblockcount",
  "lastUsedTime": 1639041504,
  "latency": {
    "max": 1,
    "min": 0,
    "avg": 0
  },
  "payload": {
    "max": 37,
    "min": 37,
    "avg": 37
  },
  "count": 3,
  "history": [
    {
      "timestamp": 1639041311,
      "latency": 0,
      "payload": 37
    },
    {
      "timestamp": 1639041348,
      "latency": 1,
      "payload": 37
    },
    {
      "timestamp": 1639041504,
      "latency": 1,
      "payload": 37
    }
  ]
}

defi-cli -regtest listrpcstats
[
  {
    "name": "getblockcount",
    "lastUsedTime": 1639041504,
    "latency": {
      "max": 1,
      "min": 0,
      "avg": 0
    },
    "payload": {
      "max": 37,
      "min": 37,
      "avg": 37
    },
    "count": 3,
    "history": [
      {
        "timestamp": 1639041311,
        "latency": 0,
        "payload": 37
      },
      {
        "timestamp": 1639041348,
        "latency": 1,
        "payload": 37
      },
      {
        "timestamp": 1639041504,
        "latency": 1,
        "payload": 37
      }
    ]
  },
  {
    "name": "getrpcstats",
    "lastUsedTime": 1639041506,
    "latency": {
      "max": 0,
      "min": 0,
      "avg": 0
    },
    "payload": {
      "max": 329,
      "min": 167,
      "avg": 248
    },
    "count": 2,
    "history": [
      {
        "timestamp": 1639041487,
        "latency": 0,
        "payload": 167
      },
      {
        "timestamp": 1639041506,
        "latency": 0,
        "payload": 329
      }
    ]
  },
  {
    "name": "listmasternodes",
    "lastUsedTime": 1639041356,
    "latency": {
      "max": 1,
      "min": 1,
      "avg": 1
    },
    "payload": {
      "max": 4255,
      "min": 4255,
      "avg": 4255
    },
    "count": 1,
    "history": [
      {
        "timestamp": 1639041356,
        "latency": 1,
        "payload": 4255
      }
    ]
  },
  {
    "name": "listrpcstats",
    "lastUsedTime": 1639041536,
    "latency": {
      "max": 0,
      "min": 0,
      "avg": 0
    },
    "payload": {
      "max": 608,
      "min": 608,
      "avg": 608
    },
    "count": 1,
    "history": [
      {
        "timestamp": 1639041536,
        "latency": 0,
        "payload": 608
      }
    ]
  },
  {
    "name": "listunspent",
    "lastUsedTime": 1639041363,
    "latency": {
      "max": 3400,
      "min": 3400,
      "avg": 3400
    },
    "payload": {
      "max": 5313266,
      "min": 5313266,
      "avg": 5313266
    },
    "count": 1,
    "history": [
      {
        "timestamp": 1639041363,
        "latency": 3400,
        "payload": 5313266
      }
    ]
  }
]

[prasannavl]

Use something on the lines of

  RPCStatEntry {
    name: string,
    lastUsedTime: int64,
    latency: RangeStatEntry { min, max, avg },
    payload: RangeStatEntry,
    count: uint64,
    history: std::vector<StatHistoryEntry>
  }
  
  std::map<string(name), RPCStatEntry> so it's easier to look up. (or unordered_map which is lookup optimized hash table, but the map will be ready for nicer json output -- but not a hotspot so doesn't really matter). 

for the internal memory repr and serialize / deserliaze in one-go.

Currently, each UniValue is (which adds up quickly)

  UniValue::VType typ;
  std::string val;                       // numbers are stored as C++ strings
  std::vector<std::string> keys;
  std::vector<UniValue> values;

[prasannavl]

The reason this PR is still pending is that, map is a protected global resource. While part of it is accessed correctly, under the lock, there are places where the references that it returns are accessed freely, which again should be protected, since it does not return a copy of the structs, but rather the references to global structures. This will end up in races, and is dependent on the higher access patterns synchronising this and serialising access.

[bvbfan]

@Jouzo rebase to v3, revert your last changes, they are incorrect.

[prasannavl]

Based on #970 (comment), the shared resource usage is well covered as it is right now.

All usages are simple value types that are copied over on the stack as stated in the above. Let's make sure there are no usage patterns that cause dead lock and we can proceed to merge.

[prasannavl]

Build failure was due to v3 branch. Should be good now.