!shutdown now has a callback /async version

[ktoso]

Motivation:

Since we noticed it again in the sample app

        system.shutdown(queue: .global(), afterShutdownCompleted: { error in
            receptacle.offerOnce(error)
        })

Modifications:

• we now have a callback on shutdown.
• the wait() can throw, which makes sense to get the errors out if we dont have the callback

Result:

resolves #230

[ktoso]

Timing sensitive Test Case 'DowningClusteredTests.test_many_nonLeaders_shouldPropagateToOtherNodes' failed (72.414 seconds) still 🤔

[ktoso]

Ticket #712