@Override
  public void open() throws IOException {
    super.open();
    referenceCount.incrementAndGet();
  }
Opens the mob file if it's not opened yet and increases the reference. It's not thread-safe.
Use MobFileCache.openFile() instead. The reader of the mob file is just opened when it's not
opened no matter how many times this open() method is invoked. The reference is a counter that
how many times this reader is referenced. When the reference is 0, this reader is closed.
hbase-server/src/main/java/org/apache/hadoop/hbase/mob/CachedMobFile.java