Bounded election buckets with dynamic reprioritization

[pwojcikdev]

Based on:

• Elections sets (can't find PR, Colin's initial attempt at bucket limiting)
• Bobs bucket vacancy branch https://github.com/nanocurrency/nano-node/compare/develop...gr0vity-dev:nano-node:wip/bucket_vacancy

but simpler, more robust and allows for dynamically dropping and scheduling higher priority elections. Also, buckets can be configured to opportunistically use more available space if AEC is underutilized.