Add batching to vote_generator

[pwojcikdev]

There was a very inefficient usage of database write transactions inside vote_generator class where we would create a new write transaction each time we wanted to generate a vote for a block. This led to stalls inside election scheduler and active transaction threads because both of those make a call to election::generate_votes().

To fix this I introduced a new class processing_queue that is a simple queue that can be configured to process batches of items. I then use this queue to process multiple voting requests at once if there is a backlog.