Add support for Aurora service announcements

Aurora zookeeper serversets are similar to what Nerve does, but Aurora
uses a different json structure in the zookeeper nodes, and it has
provisions for more than one advertised port for a service.

Change-Id: I55303bd68677706146dcfef267e625ffea1e80f3
Nix expression to build synapse from git

Change-Id: Ic521adb3989fec423cd63dc5f5ab8a58d39a5e8a