2.x: improve request accounting overhead in retry/repeat

[akarnokd]

This PR improves the request accounting overhead in the retry and repeat operators.

Previously, every individual onNext invocation signaled a an item production, which had a lot of overhead since the SubscriptionArbiter has to serialize invocations of setSubscription, request and produced. The improved algorithm counts the onNext calls in a field and calls produced once with the total count before subscribing to the upstream again.

Other small changes:

• rename inner class to RetrySubscriber (sloppy copy-paste),
• add isCancelled() check to repeatUntil subscription loop, which was somehow missing.

[codecov]

Codecov Report

Merging #5790 into 2.x will decrease coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5790      +/-   ##
============================================
- Coverage     96.29%   96.24%   -0.06%     
  Complexity     5809     5809              
============================================
  Files           634      634              
  Lines         41607    41629      +22     
  Branches       5770     5775       +5     
============================================
  Hits          40066    40066              
- Misses          614      626      +12     
- Partials        927      937      +10

Impacted Files	Coverage Δ	Complexity Δ
...ex/internal/operators/flowable/FlowableRepeat.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...nal/operators/flowable/FlowableRetryPredicate.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ternal/operators/flowable/FlowableRepeatUntil.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...l/operators/flowable/FlowableRetryBiPredicate.java	100% <100%> (ø)	2 <0> (ø)	⬇️
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-5.89%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.24% <0%> (-5.83%)	54% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	92.51% <0%> (-2.65%)	10% <0%> (-1%)
... and 28 more

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[artem-zinnatullin]

Nice!