Fix backoff when connecting to cluster with leader election in progress

[anttirt]

When the client attempts to connect to a cluster that's in the process of leader re-election, the TCP connection initially succeeds but is then closed by the ZK node. The current backoff logic treats this as a "success" and resets the backoff, resulting in reconnect attempts without any backoff, and with many clients this can result in a considerable load on the ZK servers at a very critical moment, potentially delaying the re-election and recovery.

The effect can be seen with https://github.com/anttirt/zktestcluster (you'll need Virtualbox and Vagrant) and any client connecting to it using ZooKeeperNet. Set up the cluster (vagrant up), connect a client to it and run vagrant -c ssh //vagrant/restart-leader.sh. You should see the client attempting many (dozens) of reconnects with no backoff.

This patch delays resetting the backoff until we have a confirmed session on the ZK node.