2.x: Add refCount with count & disconnect timeout

[akarnokd]

This PR exposes the additional refCount operation modes from #5975:

• Connect only when the specified number of Subscribers/Observers have subscribed
• Disconnect when the given amount of time elapsed since the very last Subscriber/Observer cancelled/disposed.
• The combination of both.

In addition, the original refCount received extra JavaDocs details.

[codecov]

Codecov Report

Merging #5986 into 2.x will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5986      +/-   ##
============================================
+ Coverage     98.22%   98.24%   +0.01%     
- Complexity     6053     6062       +9     
============================================
  Files           656      656              
  Lines         44078    44094      +16     
  Branches       6118     6118              
============================================
+ Hits          43297    43321      +24     
+ Misses          235      231       -4     
+ Partials        546      542       -4

Impacted Files	Coverage Δ	Complexity Δ
...va/io/reactivex/flowables/ConnectableFlowable.java	100% <100%> (ø)	12 <5> (+5)	⬆️
...o/reactivex/observables/ConnectableObservable.java	100% <100%> (ø)	12 <5> (+5)	⬆️
...nternal/operators/observable/ObservableCreate.java	93.04% <0%> (-5.22%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...nternal/operators/observable/ObservableReplay.java	98.08% <0%> (-0.55%)	23% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-0.55%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.65% <0%> (-0.54%)	5% <0%> (ø)
... and 15 more

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[vanniktech]

Nice

[vanniktech]

Nice

[akarnokd]

/cc @artem-zinnatullin