2.x: Fix publish().refCount() hang due to race

[akarnokd]

This PR adds a workaround implementation to the publish() implementations ObservablePublish and FlowablePublish so that when used with refCount, a connection-disconnection race won't leave consumers hanging in certain situations.

This was necessary to avoid the behavior change with the classic implementations used without refCount. In the alternate implementation, when the connectable terminates, the terminal event is signaled to late consumers until the operator is reset via the ResettableConnectable.

The refCount operator detects the classic implementations through the ObservablePublishClassic and FlowablePublishClassic internal interfaces and replaces them via the new implementations. The introduction of these interfaces were necessary to support 3rd party hooks that would intercept the classic connectable implementations via an arbitrary class which refCount wouldn't recognize or replace.

Fixes #6501

[codecov]

Codecov Report

Merging #6505 into 2.x will decrease coverage by 0.02%.
The diff coverage is 96.59%.

@@             Coverage Diff              @@
##                2.x    #6505      +/-   ##
============================================
- Coverage     98.26%   98.23%   -0.03%     
- Complexity     6295     6329      +34     
============================================
  Files           675      677       +2     
  Lines         45163    45484     +321     
  Branches       6244     6318      +74     
============================================
+ Hits          44380    44683     +303     
- Misses          248      251       +3     
- Partials        535      550      +15

Impacted Files	Coverage Δ	Complexity Δ
...o/reactivex/observables/ConnectableObservable.java	100% <100%> (ø)	14 <3> (+2)	⬆️
...ternal/operators/observable/ObservablePublish.java	98.24% <100%> (+3.55%)	12 <1> (+1)	⬆️
...va/io/reactivex/flowables/ConnectableFlowable.java	100% <100%> (ø)	14 <3> (+2)	⬆️
...x/internal/operators/flowable/FlowablePublish.java	96.65% <100%> (+0.45%)	13 <2> (+3)	⬆️
...nal/operators/observable/ObservablePublishAlt.java	96.26% <96.26%> (ø)	15 <15> (?)
...nternal/operators/flowable/FlowablePublishAlt.java	96.5% <96.5%> (ø)	17 <17> (?)
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-7.19%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	95.83% <0%> (-3.48%)	55% <0%> (-2%)
... and 28 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 89d506d...2626dd1. Read the comment docs.