Cleanup expired workers and votes on them

[abitmore]

Things to be done:

• cleanup expired workers and votes on them
• don't allow voting on expired workers

Note: now update_worker_votes() iterates through all workers. After this issue is done, it will only iterate through active workers.

Please be aware that UI may be affected: https://github.com/bitshares/bitshares-ui/pull/1350/files

More info: bitshares/bitshares-ui#1352

[oxarbitrage]

this can maybe be included here https://github.com/bitshares/bsips/blob/master/bsip-0028.md for a full worker proposal cleanup, improvment and hardfork.