AsyncJob fail when client disconnected

[LossyDragon]

Description

Originally from SK issue 1531 and SK PR 1532

AsyncJobManager shoudn't accept new jobs after SetTimeoutsEnabled(false) called.
An AsyncJob should cancel immidiatly because there is no chance to be completed(we are disconnected at this point) and also no options to be canceled because the calcellation timer is stopped.

Also added isCompletedExceptionally on most Job tests. With farily recent fixes, this seems to be working correctly for all but 2 tests.

Checklist

• Code compiles correctly
• All tests passing
• Samples run successfully
• Extended the README / documentation, if necessary

[LossyDragon]

CI runners acting up again. Second build attempt completed.

[LossyDragon]

Relay from SK release notes for this specific change:

This is technically a breaking change if you relied on this behaviour.