default void postBalanceSwitch(final ObserverContext<MasterCoprocessorEnvironment> ctx,
    final boolean oldValue, final boolean newValue) throws IOException {
  }
Called after the flag to enable/disable balancing has changed.
@param ctx      the coprocessor instance's environment
@param oldValue the previously set balanceSwitch value
@param newValue the newly set balanceSwitch value
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/MasterObserver.java