Zero poolStats in the API output
### Description

I don't see miners and hash rates in the api. 1 miner is Connected. 

### Steps to Reproduce

1. Connect to Dash Pool
2. Become Jobs from Network
3. Check /api/pools/ - zero "poolStats"

### Expected behavior

Miningcore log says:

> [2018-06-21 18:34:53.3930] [I] [dash1] [Pool] Broadcasting job 
> [2018-06-21 18:35:03.3843] [I] [dash1] [Pool] Broadcasting job 
> [2018-06-21 18:35:13.3869] [I] [dash1] [Pool] Broadcasting job 
> [2018-06-21 18:35:13.3869] [I] [dash1] [Pool] [0HLENJV7PQS9T] Booting zombie-worker (idle-timeout exceeded) 
> [2018-06-21 18:35:13.4728] [I] [dash1] [Pool] [0HLENJV7PQS9U] = < remove worker address > = <remove ip >
> [2018-06-21 18:35:23.3848] [I] [dash1] [Pool] Broadcasting job

### Reproduces how often

2 h

### Version

Postgresql 9.6.7
Docker version 17.12.0-ce, build c97c6d6
.NET Core 4.6.26515.07 on Linux 4.9.0-4-amd64 #1 SMP Debian 4.9.51-1 (2017-09-28) [X64]
Miningcore Release 24

### Additional Information

API Output:

```sh
{
  pools: [
    {
      id: "dash1",
      coin: {
        type: "DASH",
        algorithm: "X11"
      },
      ports: {
        3062: {
          listenAddress: "0.0.0.0",
          name: "ASIC Mining",
          difficulty: 1024,
          varDiff: {
            minDiff: 512,
            targetTime: 15,
            retargetTime: 90,
            variancePercent: 30
          }
        }
      },
      paymentProcessing: {
        enabled: true,
        minimumPayment: 0.5,
        payoutScheme: "PPLNS",
        payoutSchemeConfig: {
          factor: 2
        }
      },
      clientConnectionTimeout: 600,
      jobRebroadcastTimeout: 10,
      blockRefreshInterval: 500,
      poolFeePercent: 0.1,
      address: "< remove address >",
      addressInfoLink: "< remove address >",
      poolStats: {
        connectedMiners: 0,
        poolHashrate: 0,
        sharesPerSecond: 0
      },
      networkStats: {
        networkHashrate: 1693433593275836,
        networkDifficulty: 76763321.7308264,
        lastNetworkBlockTime: "2018-06-21T18:25:10.878017",
        blockHeight: 890802,
        connectedPeers: 11
      },
      topMiners: [ ],
      totalPaid: 0
    }
  ]
}
```
