Fixed endless loop and added exponential backoff to avoid DoS in case of errors.

* Fixed endless loop when etcd returns "401: The event in requested index is outdated and cleared". Fix resets the waitIndex on each List()/Get() request.
* Added exponential backoff in configBackends.WatchToUpdate() goroutine to avoid DoS on etcd/consul in event of transient problems. Maximum backoff time is 30s.
* Do note that the same problem could be possibly be present in the consul backend where a waitIndex is also used in ConsulStore.Watch().