cancelOnGracefulShutdown hangs, if cancellation is not immediately

[fabianfett]

Motivation

Currently cancelOnGracefulShutdown hangs forever, if cancellation is not immediately but returns a eventually successful result (test case 1: testCancelOnGracefulShutdownSurvivesCancellation).
If the Cancellation leads to another error than CancellationError, this error is currently not propagated to the user (test case 2: testCancelOnGracefulShutdownSurvivesErrorThrown).

Changes

• AsyncGracefulShutdownSequence should never throw as this hides the real errors otherwise
• CancellationWaiter should never throw as this hides the real errors otherwise
• Adjustments in cancelOnGracefulShutdown

Result

• cancelOnGracefulShutdown works as expected

[FranzBusch]

Nice fix! Thanks for doing the work.