[SON-HIVE Voting] Refactor maintenance block processing to distribute rewards to SONs per sidechains

The goal is to separate voting for Bitcoin and Hive SONs, so that they can act independently of each others. All code affected by this change should be refactored, in a way that enables that Bitcoin SON and Hive SON are considered separate entities.

Function of interest:

void database::pay_sons()