Uniter API now has WatchActions

This commit adds state/api and state/apiserver endpoints for
WatchActions.
I can give that a shot, but are you saying we should entirely replace the current u.st.call() approach in state/api/uniter/unit.go?  This seems like an awful lot of legwork to test something which we can logically deduce will only fail in the event of a larger failure of the RPC mechanism.

Right now our ActionsWatcher tests on State are exercising the potential for the duplicate StringsWatcherResponses and handling those errors, so if duplicates are coming down the wire, it's clearly a fault with the RPC mechanism here, which should itself be testing for such conditions.

I'm open to being wrong here, but the reason my tests are somewhat minimal on this method is that the mechanisms are being exercised in their respective packages, and this test is simply ensuring that the RPC calls are working suitably.  We're really trying to avoid spinning our wheels in the API; these PRs are backlogging work on filter, which needs to happen in order to push forward on Actions.

Again, if you think this isn't a consistent argument, we can go ahead and push the requested changes through.  I just want to be sure we're not expending significant effort here with minimal benefit when we have an obligation to move forward to get an end-to-end Action pulled together.