Add a command to compact all the regions in a regionserver

When adding a new regionserver to the hbase cluster, the data locality of the regions in the new regionserver is very low. Most read hdfs requests from these regions are remote read, not local read. So, the latency of read request to these regions is not stable.
Usually we can compact all the regions in the new regionserver to improve the data locality in offpeak.

So we add a command: compact_rs in hbase shell.