Give feedback about the shutdown

[schnapster]

Calling MagmaApi.shutdown() returns immediately and provides no feedback about the shutdown status of Magma to the user. This leaves the user in an awkward position when shutting down. How long do they have to wait until all resources are truly released?

To solve this issue

• Implement a way of letting the user know when Magma has fully shut down.
• Document it.

[shikhir-arora]

@napstr Correct me if I'm wrong, but this seems to/is partially fulfilled I believe with #12