Add total time taken to process a query in result

[mangalaman93]

Example response:

{
  "data": {
    "q": [],
    "code": "Success",
    "message": "Done",
    "uids": {
      "uid(v)": "0x2"
    }
  },
  "extensions": {
    "server_latency": {
      "parsing_ns": 50139,
      "processing_ns": 12188065,
      "encoding_ns": 5040,
      "assign_timestamp_ns": 694780,
      "total_ns": 13175112
    },
    "txn": {
      "start_ts": 14,
      "commit_ts": 15,
      "preds": [
        "1-email",
        "1-name"
      ]
    }
  }
}

This change is 

[martinmr]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain)

[manishrjain]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[pawanrawal]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved