etcd service watcher

https://github.com/coreos/etcd is "a highly-available key value store for shared configuration and service discovery", similar to zookeeper.

etcd has a REST API, and there are already Ruby libraries ( https://github.com/ranjib/etcd-ruby , https://github.com/iconara/etcd-rb ) to varying levels of maturity/completeness.

Start by reviewing Synapse's existing service-discovery entry points and the etcd REST API, then compare the Ruby libraries linked in the issue. The issue does not name files, tests, watcher behavior, or acceptance criteria, so define those requirements before implementation; done should include a working etcd-backed service watcher and tests for its behavior.

Written by the indexing model from the issue text.