protected final SyncFuture getSyncFuture(long sequence, boolean forceSync) {
    return syncFutureCache.getIfPresentOrNew().reset(sequence, forceSync);
  }
updates the sequence number of a specific store. depending on the flag: replaces current seq
number if the given seq id is bigger, or even if it is lower than existing one
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AbstractFSWAL.java