2.x: Add throttleLatest operator

[akarnokd]

This PR adds the throttleLatest operator to Observable and Flowable, also known as conflate (#4856): it is a combination of throttleFirst and sample whereby frequent items are sampled but the first item outside the sampling window will be emitted immediately:

Resolves: #4856
Replaces: #5968

[codecov]

Codecov Report

Merging #5979 into 2.x will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##               2.x    #5979     +/-   ##
==========================================
+ Coverage     98.2%   98.31%   +0.1%     
- Complexity    6052     6062     +10     
==========================================
  Files          656      658      +2     
  Lines        44077    44264    +187     
  Branches      6118     6145     +27     
==========================================
+ Hits         43287    43517    +230     
+ Misses         243      217     -26     
+ Partials       547      530     -17

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	561 <4> (+4)	⬆️
...operators/observable/ObservableThrottleLatest.java	100% <100%> (ø)	2 <2> (?)
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	539 <4> (+4)	⬆️
...nal/operators/flowable/FlowableThrottleLatest.java	100% <100%> (ø)	2 <2> (?)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-2.39%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
... and 28 more

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[mrudangit]

Thank You @akarnokd