state: move Multiwatcher type into state

This PR resolves a lot of dependency issues between state and the multiwatcher.

* api no longer depends on `state`, just the `state/multiwatcher` pacakge for the `Delta` type
* the set of dependencies of `state/multiwatcher` is signficantly smaller, it effectively only contains types which are defined for use by state.
* all the _other_ watchers live in the state package, so this change unifies the one holdout, the multiwatcher.
state: multiwatcher tests pass
fixed tests
remove unused logger
Merge branch 'master' of https://github.com/juju/juju into 216-state-move-multiwatcher
Fix unused import
Merge pull request #1137 from davecheney/216-state-move-multiwatcher

state: move Multiwatcher type into state

This PR resolves a lot of dependency issues between state and the multiwatcher.

* api no longer depends on `state`, just the `state/multiwatcher` pacakge for the `Delta` type
* the set of dependencies of `state/multiwatcher` is signficantly smaller, it effectively only contains types which are defined for use by state.
* all the _other_ watchers live in the state package, so this change unifies the one holdout, the multiwatcher.