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.
Save the time the tracker adds a new changed id and use that to compare the age of the record on the server vs the age of the local change to decide if it's server wins or client wins. Fix up various direct uses of changedIDs to use the API and make the save-to-disk lazy to avoid excessive writes. Add a test to make sure addChangedID only increases in time.
