maoling

@MikeEdgar I will take a look. Thanks for your contribution.

@MikeEdgar Sorry for our late, I'll take a look at this weekend. Thanks for your reminder

@KimRasak - The logic only affects the `queueSendMap` which some queue is empty and others is not empty. In this case, the origin logic enters into `sendNotifications()`, your fix enters...

@colvinco Thanks for your contribution. - As you said: `NPE` may be a symptom. Doing a null check is treating symptoms and not the root cause? - I don't take...

@eolivelli Thanks for your review. - You are right,we should make the UT simple. - But IMO the configuration for the startup of zk server(e.g `myid`,`sessionTimeOut`) must really use the...

A bad news: lots of flaky tests caused by this change,Let me take another look.

- @lvfangmin the `validateServerId` logic in `EphemeralType` is not a good place. e.g if the server which `myid=255` and her role is not the leader,it can create the ttl node...

@anmolnar Yes,Let me take another look.

@ravowlga123 I guess you may also interested in [ZOOKEEPER-3649](https://issues.apache.org/jira/browse/ZOOKEEPER-3649) :)

@ericlee123 this PR needs another rebase