default void preFlush(final ObserverContext<? extends RegionCoprocessorEnvironment> c,
    FlushLifeCycleTracker tracker) throws IOException {
  }
Called before the memstore is flushed to disk.
@param c       the environment provided by the region server
@param tracker tracker used to track the life cycle of a flush
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/RegionObserver.java