CLONE - Race conditions in LeaderLatch after reconnecting to ensemble

Clone from CURATOR-504.

We use LeaderLatch in a lot of places in our system and when ZooKeeper ensemble is unstable and clients are reconnecting to logs are full of messages like the following:

[2017-08-31 19:18:34,562][ERROR][org.apache.curator.framework.recipes.leader.LeaderLatch] Can't find our node. Resetting. Index: -1 {}

According to the implementation, this can happen in two cases:

• When internal state `ourPath` is null
• When the list of latches does not have the expected one.

I believe we hit the first condition because of races that occur after client reconnects to ZooKeeper.

• Client reconnects to ZooKeeper and LeaderLatch gets the event and calls reset method which set the internal state (`ourPath`) to null, removes old latch and creates a new one. This happens in thread "Curator-ConnectionStateManager-0".
• Almost simultaneously, LeaderLatch gets another even NodeDeleted (here) and tries to re-read the list of latches and check leadership. This happens in the thread "main-EventThread".

Therefore, sometimes there is a situation when method `checkLeadership` is called when `ourPath` is null.