2.x: Introduce property rx2.scheduler.use-nanotime (#7154)

[SergejIsbrecht]

Fixes #7169

[SergejIsbrecht]

@akarnokd , I had to create a new SchedulerTest class, because on the 2.x branch the package did not match the package of Scheduler, therefore I could not access the field (package-private).

[codecov]

Codecov Report

Merging #7170 (2c46d3e) into 2.x (f31aed3) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.x    #7170      +/-   ##
============================================
+ Coverage     98.18%   98.26%   +0.08%     
- Complexity     6345     6349       +4     
============================================
  Files           677      677              
  Lines         45553    45557       +4     
  Branches       6342     6342              
============================================
+ Hits          44724    44765      +41     
+ Misses          263      254       -9     
+ Partials        566      538      -28     

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/schedulers/Schedulers.java	100.00% <ø> (ø)	11.00 <0.00> (ø)
src/main/java/io/reactivex/Scheduler.java	100.00% <100.00%> (ø)	13.00 <4.00> (+2.00)
...tivex/internal/schedulers/TrampolineScheduler.java	96.10% <0.00%> (-2.60%)	6.00% <0.00%> (ø%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0.00%> (-2.25%)	60.00% <0.00%> (ø%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0.00%> (-1.45%)	2.00% <0.00%> (ø%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0.00%> (-1.39%)	57.00% <0.00%> (-1.00%)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0.00%> (-1.06%)	3.00% <0.00%> (ø%)
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <0.00%> (-0.86%)	2.00% <0.00%> (ø%)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0.00%> (-0.56%)	3.00% <0.00%> (ø%)
...perators/mixed/ObservableConcatMapCompletable.java	99.24% <0.00%> (ø)	3.00% <0.00%> (ø%)
... and 19 more

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