private static long getLastTxid(Deque<FSWALEntry> queue) {
    return queue.peekLast().getTxid();
  }
This method is to adapt {@link FSHLog} and {@link AsyncFSWAL}. For {@link AsyncFSWAL}, we use
{@link AbstractFSWAL#highestProcessedAppendTxid} at the point we calling
{@link AsyncFSWAL#doWriterSync} method as successful syncedTxid. For {@link FSHLog}, because we
use multi-thread {@code SyncRunner}s, we used the result of {@link CompletableFuture} as
successful syncedTxid.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AbstractFSWAL.java