poolStats in /pools API call returns lastPoolBlockTime as null at all times
### Description
The /pools or /pools/{poolid} call gives the poolStats object.The lastPoolBlockTime is always null even though the pool did find a block recently

### Steps to Reproduce
- Mine some blocks on miningcore pool
- Call /pools or /pools/{poolid}

### Expected behavior

- lastPoolBlocktime should be the time the last block was mined on pool
### Actual behavior

- lastPoolBlocktime is null at all times.

### Reproduces how often

100%
### Version
Currently using my fork of miningcore,master branch https://github.com/akshaynexus/miningcore/commit/5f47bb62edfa8d22999d74c1f60400380df1ecfb

### Additional Information

