Engines now maintain a reference to the service they belong to. This
allows them to obtain references to other engine instances belonging to
that service and that service only.
Stores and trackers now maintain a reference to the engine they belong
to.
Engine managers now maintain a reference back to a service.
The clients singleton has been removed. It now exists as an instance
variable on Service. Parts of ClientsEngine do behave as singletons
(e.g. commands). This will be addressed in future refactoring.
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