Dispatch service to retrieve values from remote calls.

The Courier provides the main implementation for fallbacks triggered by calls to the fetch() command. It acts as a synchronized execution for tasks to avoid duplicating calls when loading.

The Courier uses a very simple algorithm to determine when to execute a fallback, so there's very little overhead to synchronizing calls through it. As tasks are dispatched via spawned processes, there's very little action actually happening in the service process itself.