fix locking in consensus package

There were a few locking bugs in the consensus package, places where the database was being accessed outside of a read lock.

There was one critical place in particular, which was using a readlock followed by a writelock, when it really needed to be using a single writelock -> demotelock. This place is likely the cause of the consensus desynchronizations between the miner + consensus package, has likely impacted other modules, and since fixing it I have not been able to trigger the behavior in question.