etcd: require etcd 2.3 and use new ttl refresher api

[schmichael]

etcd 2.3 added a new API for refreshing TTLs without notifying watchers. This would dramatically cut down on the number of etcd events every single metafora node has to process.

https://github.com/coreos/etcd/releases/tag/v2.3.0