rpc: Add `getmemoryinfo` call

```
getmemoryinfo
Returns an object containing information about memory usage.

Result:
{
  "locked": {               (json object) Information about locked memory manager
    "used": xxxxx,          (numeric) Number of bytes used
    "free": xxxxx,          (numeric) Number of bytes available in current arenas
    "total": xxxxxxx,       (numeric) Total number of bytes managed
    "locked": xxxxxx,       (numeric) Amount of bytes that succeeded locking. If this number is smaller than total, locking pages failed at some point and key data could be swapped to disk.
  }
}

Examples:
> bitcoin-cli getmemoryinfo
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "getmemoryinfo", "params": [] }' -H 'content-type: text/plain;' http://127.0.0.1:8332/
```

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