WIP State level WatchInterfaces

Wraps the State level WatchInterfaces watcher, and maps the Ids we get from state into Tags for the API.

This also includes exposing the WatchInterfaces call in the API, so touching state/apiserver and state/api
"have no state to transmit" seems a bit misleading to me. They have no *data*, but they're absolutely notifying of a state change (in this case, from "we don't know anything" to "we've found a base state and will henceforth transmit changes relative to that").