default InternalScanner preFlush(ObserverContext<? extends RegionCoprocessorEnvironment> c,
    Store store, InternalScanner scanner, FlushLifeCycleTracker tracker) throws IOException {
    return scanner;
  }
Called before a Store's memstore is flushed to disk.
@param c       the environment provided by the region server
@param store   the store where flush is being requested
@param scanner the scanner over existing data used in the memstore
@param tracker tracker used to track the life cycle of a flush
@return the scanner to use during flush. Should not be {@code null} unless the implementation
        is writing new store files on its own.
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/RegionObserver.java