default InternalScanner preMemStoreCompactionCompact(
    ObserverContext<? extends RegionCoprocessorEnvironment> c, Store store, InternalScanner scanner)
    throws IOException {
    return scanner;
  }
Called before we do in memory compaction. Notice that this method will only be called when you
use {@code eager} mode. For {@code basic} mode we will not drop any cells thus there is no
{@link InternalScanner}.
@param c       the environment provided by the region server
@param store   the store where in memory compaction is being executed
@param scanner the scanner over existing data used in the memstore segments being compact
@return the scanner to use during in memory compaction. Must be non-null.
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/RegionObserver.java