3.x: Fix delay-cancellation race producing random subsequent emissions

[akarnokd]

When a sequence using delay is cancelled asynchronously, there is a race between cancelling each individual Future inside the executor and them running, causing skips in the onNext emissions.

For example, let's say there are 3 futures waiting to be executed. The Future-1 triggers the cancellation asynchronously and emits 1. This asynchronous routine will start cancelling Future-2 and Future-3 one by one. Concurrently, the executor will look at Future-2, see it cancelled and moves onto Future-3, which is still executable as the asynchronous routine hasn't gotten to cancel Future-3. Now Future-3 runs and emits 3. This will appear to produce a sequence of [1, 3] where 2 is missing.

The fix is to hard-check for a disposed worker and prevent the onNext emissions in the operator.

Resolves #7851

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.62%. Comparing base (9b55d01) to head (35eb3b9).
Report is 147 commits behind head on 3.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                3.x    #7855      +/-   ##
============================================
- Coverage     99.62%   99.62%   -0.01%     
  Complexity     6801     6801              
============================================
  Files           752      752              
  Lines         47707    47715       +8     
  Branches       6401     6404       +3     
============================================
+ Hits          47527    47534       +7     
+ Misses           84       80       -4     
- Partials         96      101       +5     

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