long getUnflushedEntriesCount() {
    long highestSynced = this.highestSyncedTxid.get();
    long highestUnsynced = this.highestUnsyncedTxid;
    return highestSynced >= highestUnsynced ? 0 : highestUnsynced - highestSynced;
  }
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