Expose last share timestamp for each miner / worker in API

[mkrufky]

This is a re-send of PR #3 , as per @miketout 's request. We believe this PR was closed by accident.

---

It's useful in a UI to have the timestamp of the last share for every miners' workers.
The current s-nomp API does not expose this information, but it is stored in redis.
Let's expose it in the API.

Until now, I had to fudge the "last share" data for my pool, each worker seemed to be in sync:

After applying this patch, I have better detail in the API:

     {
        "RRNHfxjNaJ9zC7FYcYCVPuXLPpk9Vs3hFu.voskcoin": {
          "lastShare": 1563672761500,
          "name": "RRNHfxjNaJ9zC7FYcYCVPuXLPpk9Vs3hFu.voskcoin",
          "diff": 2663892268,
          "shares": 8129554.041592247,
          "invalidshares": 325182.16166369,
          "currRoundShares": 8249087416.608933,
          "currRoundTime": 233703.447,
          "hashrate": 116387229132344.4,
          "hashrateString": "232.77 MH/s",
          "luckDays": "1.452",
          "luckHours": "34.859",
          "paid": 0,
          "balance": 0
        },
        "RRNHfxjNaJ9zC7FYcYCVPuXLPpk9Vs3hFu.voskcoin2": {
          "lastShare": 1563672772522,
          "name": "RRNHfxjNaJ9zC7FYcYCVPuXLPpk9Vs3hFu.voskcoin2",
          "diff": 281319519,
          "shares": 824178.2783126401,
          "invalidshares": 0,
          "currRoundShares": 630672034.5685258,
          "currRoundTime": 233703.447,
          "hashrate": 11799395838087.918,
          "hashrateString": "23.60 MH/s",
          "luckDays": "14.327",
          "luckHours": "343.846",
          "paid": 0,
          "balance": 0
        }
      }

Pool UI now shows individual share times from each worker:

screenshots taken from the VerusCoin mining pool at verus.wattpool.net

upstream pr: s-nomp#144

[Oink70]

The last share timestamp data has been merged in #3 . This PR can be closed.