Add graceful service shutdown

[tzoiker]

Sometimes it is desirable to create tasks within a service that will either be awaited or cancelled upon service exit. Added

• GracefulMixin - creates and stores tasks for cancellation or waiting;
• GracefulService - performs graceful shutdown on stop.

[mosquito]

Please fix tests and rebase.

[tzoiker]

@mosquito is it better for create_graceful_task to have cancel=True by default? Decided to make it cancel=False.