Mutation Queues - Cursor dropping

If a cursor is slow then the mutation manager overhead will continue to grow until the server runs out of space and can cause the entire cluster to become slow. As a result we need a way to relieve memory pressure by dropping slow cursors. When we drop a cursor then this means that we have to schedule a disk backfill in order to read the items that were dropped off of disk. We also need to make sure that when we catch back up to mutations that are contained in the mutation manager that we stop reading off of disk and start reading out of memory again.