2.x: Fix bounded replay() memory leak due to bad node retention

[akarnokd]

This PR fixes a memory leak caused by node retention in the bounded replay() implementations. When the consumer disposed/canceled the flow, the index field was not cleared. In certain situations, such out-of-comission consumers were still reachable via thread local stack and through it, the node they were last refencing. Further items through it would then keep referencing an ever increasing number of linked nodes, causing OOME eventually.

The fix is to clear the index field upon dispose/cancellation. The subject/processor variants were working properly. Tests were added to verify them though as well.

Issue discovered on StackOverflow: https://stackoverflow.com/q/54191190/61158

[codecov]

Codecov Report

Merging #6371 into 2.x will increase coverage by 0.03%.
The diff coverage is 80%.

@@             Coverage Diff              @@
##                2.x    #6371      +/-   ##
============================================
+ Coverage     98.24%   98.28%   +0.03%     
- Complexity     6282     6287       +5     
============================================
  Files           673      673              
  Lines         45023    45028       +5     
  Branches       6226     6226              
============================================
+ Hits          44235    44257      +22     
+ Misses          248      243       -5     
+ Partials        540      528      -12

Impacted Files	Coverage Δ	Complexity Δ
...nternal/operators/observable/ObservableReplay.java	98.63% <100%> (-0.55%)	20 <0> (ø)
...ex/internal/operators/flowable/FlowableReplay.java	94.09% <66.66%> (-0.18%)	20 <0> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	93.8% <0%> (-4.43%)	10% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-2.18%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
...x/internal/operators/flowable/FlowableGroupBy.java	95.51% <0%> (-0.57%)	3% <0%> (ø)
... and 24 more

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[artur-jablonski]

Thank you for fixing this! You guys rock!