Decouple AbstractContinuation and CancellableContinuation from Job interface

[qwwdfsad]

Decouples CancellableContinuation from Job.

Pros:

1. Some workarounds from Job may be removed
2. State machine of AbstractContinuation becomes much simpler
3. AbstractContinuation state is now more compact and start/cancel produce less garbage (and state transitions are more lightweight)

2, 3 => all integration modules become faster (~25% win in cancellation benchmark).

Cons:

1. Binary compatibility may be broken in some places
2. Compatibility is broken due to changes in invokeOnCompletion contract
3. Small maintenance mess ("similar, but not the same" code)

In general, it boosts the performance of cancellable continuations, allows to do specific optimizations (e.g., it makes sense to get rid of resumeMode after merging this change and replace it with inheritance to reduce memory footprint even more) and allows us to move CancellableContinuation to stdlib

[fvasco]

Release with #311

[qwwdfsad]

Fixes #219