Uniter API now has WatchActions

This commit adds state/api and state/apiserver endpoints for
WatchActions.
Hmm, that MustErr is a problem. Is it the same everywhere? Even if a single watcher is somehow screwed up, it shouldn't be able to take out the whole API server...

Wouldn't surprise me if we had the damn things everywhere, so don't feel obliged if it's consistent -- but please do add a bug pointing out that MustErr is not safe and should basically never be used except in client code.