2.x: Fix the error/race in Observable.repeatWhen due to flooding repeat signal

[akarnokd]

This PR fixes a race condition in the ObservableRepeatWhen operator for the case when the repeat signal is not 1-for-1 and a new subscription may race with the clearing of the previous disposable because active is set to false too early. The fix is to swap the two operations.

Observable.retryWhen and the Flowable versions do not have this bug. Unit tests were added to all 4 operators to verify the correct behavior regardless.

Fixes: #6358

[codecov]

Codecov Report

Merging #6359 into 2.x will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6359      +/-   ##
============================================
+ Coverage     98.24%   98.26%   +0.01%     
- Complexity     6281     6283       +2     
============================================
  Files           672      672              
  Lines         44990    44990              
  Branches       6223     6223              
============================================
+ Hits          44202    44210       +8     
+ Misses          247      241       -6     
+ Partials        541      539       -2

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/observable/ObservableRepeatWhen.java	100% <100%> (+1.56%)	2 <0> (ø)	⬇️
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...perators/mixed/ObservableConcatMapCompletable.java	97.74% <0%> (-2.26%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	97.02% <0%> (-1.71%)	11% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.28% <0%> (-1.42%)	3% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	93.46% <0%> (-1.31%)	2% <0%> (ø)
... and 26 more

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[eleventigers]

Nice! Thanks @akarnokd for such a quick fix, I have verified that it fixes #6358 locally.