EC: Add normal and low priority to replication supervisor and commands

In order to allow replication commands to fix under and mis-replication to run with higher priority that replication commands related to the balancer, we will change the replication supervisor to be a priority queue, ordered by priority, enqueue time.

Commands sent from the balancer will have a low priority, other commands will a normal priority. This means that balancer commands will not run while there are other replication commands present in the queue.

This means it is not important if the balancer adds a lot of replication commands to the queue and then some nodes go down in the cluster. The replication commands related to the down nodes will automatically get to the front of the queue.