Investigate whether we still need BeforeResumeCancelHandler

[qwwdfsad]

This is an internal API that has been mostly unused since #3084 except for one place.

More codepath to check, more code to test and maintain, more regressions to happen (as the code-path is highly-specific), we should investigate whether it is possible to get rid of it

[qwwdfsad]

Fixed.
Unfortunately, there is one more allocation left that creates a difference in Channel() and Channel {} characteristics -- resumedState allocation that has non-null onCancellation lambda when used with iterator (hasNextOnNoWaiterSuspend)