router: Fix sync with pruned etcd instances.

If I start using the router with an existing etcd instance, where the router keys do not yet exists, I end up in a 

    2014/09/02 13:02:46 Got etcd error 401, doing full sync

loop. The reason is that ``nextIndex`` starts at 1, and as soon as that index has been pruned, watching with that index will always fail.