Distinguish read and write request count in region

Distinguishing read and write request counts, on top of HBASE-3507, would benefit load balancer.
The action for balancing read vs. write load should be different. For read load, region movement should be low (to keep scanner happy). For write load, region movement is allowed.
Now that we have cheap(er) counters, it should not be too burdensome keeping up the extra count.