Move balancer threads count in config options.

Mover threads (which move blocks) and dispatcher threads (which submits
requestst to movers) can produce significant NN load. To have ability to run
balancer in nice mode, amount of these threads are now can be provided in config file.

The options are dfs.balancer.moverThreads and dfs.balancer.dispatcherThreads.
Merge branch 'hbase-balancer'