Rethink data module / store interface

[squaremo]

Most of the time, we just want a list or a map of the services or instances of a service. In the case of a list, we also usually want the service name to be part of each item -- after all, the only guaranteed property of a service is its name.

So: we should make the name part of the in-memory representation of a service; and, the store could just return lists or maps of services and instances.

[squaremo]

Done in c30499d64eaa8157ad55ec3701219b19dea470b6