@Override
  protected int getStateId(GCRegionState state) {
    return state.getNumber();
  }
GC a Region that is no longer in use. It has been split or merged away. Caller determines if it
is GC time. This Procedure does not check.
<p>
This is a Region StateMachine Procedure. We take a read lock on the Table and then exclusive on
the Region.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/assignment/GCRegionProcedure.java