Improve pay_workers() performance

- [ ] This code iterates through all worker objects:
https://github.com/bitshares/bitshares-core/blob/06aee789a4e2045d899e02b096d5a1a908be2865/libraries/chain/db_maint.cpp#L120-L125

It can be optimized with a new `by_expiration` index.

This is low priority because we don't have too many workers so far.

- [ ] This code can be replaced with a `by_votes` index, I'm not sure the performance would be better or not though:
https://github.com/bitshares/bitshares-core/blob/06aee789a4e2045d899e02b096d5a1a908be2865/libraries/chain/db_maint.cpp#L127-L135

Start in libraries/chain/db_maint.cpp at lines 120-125 and trace the worker object indexes used by pay_workers(). Evaluate adding a by_expiration index so the function avoids iterating through all workers; done means worker payouts retain their behavior while the relevant lookup is more efficient.

Written by the indexing model from the issue text.