Add ZookeeperDnsWatcher
always use the watcher's `reconfigure!` method

consistency between watchers
Improve check interval queuing behavior, add comments
Address comments from PR

* Make ping? dependent on watcher thread
* Fix default_servers handling
Add slightly more logging
Merge pull request #65 from airbnb/add-zookeeper-dns-watcher

WIPish: Add ZookeeperDnsWatcher