2.x: fix buffer(time, maxSize) duplicating buffers on time-size race

[akarnokd]

The PR fixes both the time+maxSize bound buffer operators of Flowable and Observable. The logic didn't properly mutually exclude the timer action and the onNext action, resulting in probabilistic emission of the same buffer twice.

Reported in #5426.

[codecov]

Codecov Report

Merging #5427 into 2.x will increase coverage by 0.16%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5427      +/-   ##
============================================
+ Coverage     95.88%   96.05%   +0.16%     
- Complexity     5789     5792       +3     
============================================
  Files           631      631              
  Lines         41297    41291       -6     
  Branches       5742     5742              
============================================
+ Hits          39599    39661      +62     
+ Misses          679      633      -46     
+ Partials       1019      997      -22

Impacted Files	Coverage Δ	Complexity Δ
...al/operators/observable/ObservableBufferTimed.java	92.19% <100%> (+0.28%)	5 <0> (ø)	⬇️
...ternal/operators/flowable/FlowableBufferTimed.java	90.84% <100%> (-0.1%)	5 <0> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	93.87% <0%> (-6.13%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.66% <0%> (-5.56%)	9% <0%> (-1%)
...ternal/operators/completable/CompletableUsing.java	95.23% <0%> (-4.77%)	4% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	91.3% <0%> (-2.72%)	2% <0%> (ø)
...ava/io/reactivex/internal/util/HalfSerializer.java	97.5% <0%> (-2.5%)	22% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-2.5%)	2% <0%> (ø)
... and 37 more

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