3.x: Fix refCount not resetting when termination triggers cross-cancel

[akarnokd]

This PR fixes both refCount implementation to properly reset the source when the source terminates multiple consumers.

In the original, when the source terminated multiple sources, the first termination handler cleared the connection but since the --subscriberCount wasn't zero, the source was not reset. If this termination triggered a cancel on the second consumer, that path would not get the source reset either due to losing the connection object. Unfortunately, one can't just take the first termination handler and reset there immediately either because that could disrupt the termination of the rest of the consumers. It has to wait for the source to terminate all consumers or have the consumers cancel all.

This affects 2.x and will be backported in a separate PR.

Fixes #6608

[codecov]

Codecov Report

Merging #6609 into 3.x will increase coverage by 0.06%.
The diff coverage is 80%.

@@             Coverage Diff             @@
##                3.x   #6609      +/-   ##
===========================================
+ Coverage     98.23%   98.3%   +0.06%     
+ Complexity     6185    6181       -4     
===========================================
  Files           678     678              
  Lines         44908   44910       +2     
  Branches       6214    6214              
===========================================
+ Hits          44117   44150      +33     
+ Misses          261     251      -10     
+ Partials        530     509      -21

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/observable/ObservableRefCount.java	98.97% <80%> (-1.03%)	22 <0> (-2)
.../internal/operators/flowable/FlowableRefCount.java	98.98% <80%> (-1.02%)	22 <0> (-2)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.68% <0%> (-0.97%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	99.09% <0%> (-0.91%)	17% <0%> (-1%)
...nternal/operators/parallel/ParallelSortedJoin.java	94.89% <0%> (-0.73%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowableGroupBy.java	95.51% <0%> (-0.57%)	3% <0%> (ø)
...activex/internal/schedulers/ExecutorScheduler.java	98% <0%> (-0.5%)	10% <0%> (ø)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.28%)	3% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	98.52% <0%> (ø)	17% <0%> (-1%)	⬇️
... and 22 more

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