[3.x] Consistent cancellation semantics for coroutine()

[clue]

This changeset ensures we're using consistent cancellation semantics for coroutine(). In particular, calling cancel() on the resulting promise will now try to cancel the first pending operation only. Cancelling a pending operation would usually throw an exception and thus reject the resulting promise. If this exception in caught inside the coroutine and another operation is started or if the pending operation does not support cancellation, the coroutine may continue executing.

Refs #42
Builds on top of #12, #13 and #47