2.x: fix SpscLAQ nepotism, FlowableRefCountTest.testRefCountAsync flaky

[akarnokd]

This PR fixes the so-called GC Nepotism (see #3794) in SpscLinkedArrayQueue by not nulling out the previous buffer's next pointer upon switching buffers in the peek()/poll() methods.

In addition, the FlowableRefCountTest.testRefCountAsync has been reworked to be more forgiving towards unexpected delays due to system load (reported in #5506).

[codecov]

Codecov Report

Merging #5507 into 2.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5507      +/-   ##
============================================
- Coverage     96.04%   96.02%   -0.03%     
+ Complexity     5808     5797      -11     
============================================
  Files           631      631              
  Lines         41282    41285       +3     
  Branches       5732     5732              
============================================
- Hits          39651    39645       -6     
+ Misses          649      647       -2     
- Partials        982      993      +11

Impacted Files	Coverage Δ	Complexity Δ
...reactivex/internal/queue/SpscLinkedArrayQueue.java	96.82% <100%> (-0.74%)	38 <1> (-1)
...tivex/internal/schedulers/InstantPeriodicTask.java	61.11% <0%> (-5.56%)	7% <0%> (-2%)
...ternal/operators/flowable/FlowableSampleTimed.java	91.17% <0%> (-4.42%)	3% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...rnal/operators/flowable/FlowableTakeLastTimed.java	96.29% <0%> (-2.78%)	2% <0%> (ø)
...java/io/reactivex/subjects/CompletableSubject.java	94.8% <0%> (-2.6%)	35% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...main/java/io/reactivex/subjects/SingleSubject.java	97.61% <0%> (-2.39%)	39% <0%> (-1%)
.../main/java/io/reactivex/subjects/MaybeSubject.java	95.65% <0%> (-2.18%)	46% <0%> (-1%)
...activex/internal/schedulers/ExecutorScheduler.java	94.59% <0%> (-2.03%)	9% <0%> (ø)
... and 41 more

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