default void postMergeRegions(final ObserverContext<MasterCoprocessorEnvironment> c,
    final RegionInfo[] regionsToMerge) throws IOException {
  }
called after merge regions request.
@param c              coprocessor environment
@param regionsToMerge regions to be merged
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java