ZOOKEEPER-1011:fix Java Barrier Documentation example's race condition issue and polish up the Barrier Documentation

[maoling]

• This patch fix the race condition of double barrier and avoid the herd effect.
• To focus on the core logic and keep the codes short and clean,don't consider some parameters check,connect loss,the timeout for wait ....etc.The thoughtful codes can be found in the implements of DistributedDoubleBarrier of curator-recipes.
• For test,you can change the QTY in the barrierTest from small one(1,2,3) to large one(1000,5000,10000),then observer the printed log to check the correctness of this patch.
• more details in ZOOKEEPER-1011 .Thanks for
  semihsalihoglu reporting this issue.

[nameof]

considering spurious wakeup, you should always use while(true) here, or change if (!hasBeenNotified.get()) to while (!hasBeenNotified.get())
https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--

[maoling]

@nameof
you can test this code by printing some logs to verify the correctness（wrong） of what you think（hahaha）