Draft: Adds Taskscaler's shutdown signaling

When the provided context.Context is closed, Taskscaler is supposed to call Fleeting provisioner's Shutdown() method and wait for it to be done. But as everything happens in a dedicated goroutine, it's impossible to the consumer to know when it was finalized.

This change adds a mechanism to signal the consumer that provisioner's shutdown is compleeted and that the taskscaler have exited its main loop.