void prepare(T instance) {
    assert instance != null;
    instanceHolder = new InstanceHolder<>(instance);
    pendingLatch.countDown();
  }
Associates the given instance for the method {@link #get}. This method should be called once,
and {@code instance} should be non-null. This method is expected to call as soon as possible
because the method {@code get} is uninterruptibly blocked until this method is called.
hbase-zookeeper/src/main/java/org/apache/hadoop/hbase/zookeeper/InstancePending.java