[FEATURE] Added Pool Bonus for Prop and PPLNS

Prop and PPLNS will now allow a pool operator to run a pool bonus. It is based on the block reward and evenly paid as a percentage (default `0`) to all miners.

PPS will pose a challenge since our block bonus is block bound (confirmations) and PPS does not care about blocks. So we can not easily bind a PPS pool bonus to a block.

Since PPS is not widely used, I think this a minor issue for now. Please test the Prop and PPLNS implementations, they worked fine on my testnet.

Fixes #1932 once merged.