HBASE-6028 Start/Stop compactions at region server level
Add switching on/off of compactions.
Switching off compactions will also interrupt any currently ongoing compactions.
Adds a "compaction_switch" to hbase shell. Switching off compactions will
interrupt any currently ongoing compactions. State set from shell will be
lost on restart. To persist the changes across region servers modify
hbase.regionserver.compaction.enabled in hbase-site.xml and restart.