**Service Dependencies**
Services that depend on other services now access those services via the `Invoker` object. This object is provided to the service as a kwarg to its `start()` method.
Until now, most services did not utilize this feature, and several services required their dependencies to be initialized and passed in on init.
Additionally, _all_ services are now registered as invocation services - including the low-level services. This obviates issues with inter-dependent services we would otherwise experience as we add workflow storage.
**Database Access**
Previously, we were passing in a separate sqlite connection and corresponding lock as args to services in their init. A good amount of posturing was done in each service that uses the db.
These objects, along with the sqlite startup and cleanup logic, is now abstracted into a simple `SqliteDatabase` class. This creates the shared connection and lock objects, enables foreign keys, and provides a `clean()` method to do startup db maintenance.
This is not a service as it's only used by sqlite services.
