Region IPC operations should be high priority

I manufactured an imbalanced cluster so one region server had 300 regions and the others had very few. I then ran balancer while hitting the high-load region server with YCSB. I observed that the rate of load shedding was VERY slow since the closeRegion IPCs were getting stuck at the back of the IPC queue.

All of these important master->RS RPC calls should be set to high priority.