Replace Server.IsRunning with Engine.IsShutdown

Signed-off-by: Solomon Hykes <solomon@docker.com>
Extra testing for engine.Shutdown

Signed-off-by: Solomon Hykes <solomon@docker.com>
Remove last trace of Daemon->Server dependency

This removes a shim `daemon.Server` interface which was used to start
separating Daemon from Server *gradually*, without getting cyclic
dependency errors.

Now that the last Daemon->Server dependency has been removed, we can
finally remove the shim. Yay!

Signed-off-by: Solomon Hykes <solomon@docker.com>
Remove unused field from engine.Job

Signed-off-by: Solomon Hykes <solomon@docker.com>
Jobs can be stopped (cooperatively) with job.Stop() and job.OnStop()

Signed-off-by: Solomon Hykes <solomon@docker.com>
Engine.Shutdown tries to stop all jobs before waiting for them to retun

Signed-off-by: Solomon Hykes <solomon@docker.com>