Clients was being exported and used as a singleton. We still use Clients
as a singleton in some places, but only in test code. The preferred
method to access Clients is now through a service instance.
Weave.Clients is no longer exposed. Callers go through Weave.Service
now.
Weave.Engines is no longer exported. Service now exposes an
EngineManager instance, which is the new recommended way to get at the
engine manager. Service was updated to reference the internal instance.
