3.x: Sync up with 2.2.10 snapshot

[akarnokd]

The Changes.md always comes up conflicting. I'll leave it as it is for now.

[vanniktech]

Why are we pushing for 3.x?

[akarnokd]

I decided it is time to move forward towards the next major release.

[codecov]

Codecov Report

Merging #6507 into 3.x will decrease coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               3.x    #6507      +/-   ##
===========================================
- Coverage     98.3%   98.26%   -0.04%     
  Complexity    6296     6296              
===========================================
  Files          675      675              
  Lines        45163    45165       +2     
  Branches      6244     6246       +2     
===========================================
- Hits         44397    44383      -14     
- Misses         236      241       +5     
- Partials       530      541      +11

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	567 <0> (ø)	⬇️
.../main/java/io/reactivex/schedulers/Schedulers.java	100% <ø> (ø)	11 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	542 <0> (ø)	⬇️
...ternal/operators/flowable/FlowableBufferTimed.java	99.27% <100%> (ø)	5 <0> (ø)	⬇️
...al/operators/observable/ObservableBufferTimed.java	99.62% <100%> (ø)	5 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <0%> (-5.89%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	93.16% <0%> (-5.13%)	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%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
... and 25 more

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