2.x: fix ReplayProcessor backpressure and NotificationLite emission bug

[akarnokd]

This PR upgrades the replaying algorithms of the ReplayProcessor to the standard queue-drain and as an added effect, fixes a request-emission race where the internal NotificationLite.complete could be emitted to the consumers.

[codecov]

Codecov Report

Merging #5493 into 2.x will increase coverage by 0.12%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5493      +/-   ##
============================================
+ Coverage     96.11%   96.23%   +0.12%     
- Complexity     5798     5809      +11     
============================================
  Files           631      631              
  Lines         41297    41276      -21     
  Branches       5743     5730      -13     
============================================
+ Hits          39692    39723      +31     
+ Misses          632      602      -30     
+ Partials        973      951      -22

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/flowable/FlowableTimeoutTimed.java	93.22% <ø> (ø)	4 <0> (ø)	⬇️
.../java/io/reactivex/processors/ReplayProcessor.java	99.35% <100%> (+0.58%)	53 <8> (+4)	⬆️
...tivex/internal/schedulers/InstantPeriodicTask.java	61.11% <0%> (-5.56%)	7% <0%> (-2%)
.../operators/observable/ObservableIntervalRange.java	97.36% <0%> (-2.64%)	3% <0%> (ø)
...main/java/io/reactivex/subjects/SingleSubject.java	97.61% <0%> (-2.39%)	39% <0%> (-1%)
...ternal/operators/completable/CompletableUsing.java	97.61% <0%> (-2.39%)	4% <0%> (ø)
.../main/java/io/reactivex/subjects/MaybeSubject.java	95.65% <0%> (-2.18%)	46% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
...al/operators/observable/ObservableSampleTimed.java	96.66% <0%> (-1.67%)	3% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	93.28% <0%> (-1.5%)	2% <0%> (ø)
... and 35 more

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