Add nextNetworkBits and nextNetworkTarget to /api/pools

[bitspill]

Adds the bits/target of the next network block to the pool.networkStats section of the api/pools API

These additional datapoints were necessary for a FLO application but could perhaps be generally useful for debugging or other mining related analysis

Example:

      "networkStats": {
        "networkType": "Test",
        "networkHashrate": 10547.259085580305,
        "networkDifficulty": 0.000244137132,
        "nextNetworkTarget": "00000fffff000000000000000000000000000000000000000000000000000000",
        "nextNetworkBits": "1e0fffff",
        "lastNetworkBlockTime": "2018-10-25T15:41:49.5252676Z",
        "blockHeight": 254062,
        "connectedPeers": 16,
        "rewardType": "POW"
      },