1.x: throttleFirst detecting clock-drift backwards to open the gate

[akarnokd]

If the current time moves backwards before the time the gate was closed, the opening of the gate would happen way later. Since we don't know how much relative time has passed, this fix opens the gate when the drift is detected during onNext.

Reported in #5120.

/cc @davidmoten as you've been experimenting with time-only (that is non-scheduled) gating.

[codecov-io]

Codecov Report

Merging #5123 into 1.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                1.x    #5123      +/-   ##
============================================
+ Coverage     84.38%   84.41%   +0.02%     
- Complexity     2876     2877       +1     
============================================
  Files           290      290              
  Lines         18070    18070              
  Branches       2468     2468              
============================================
+ Hits          15249    15254       +5     
- Misses         1951     1954       +3     
+ Partials        870      862       -8

Impacted Files	Coverage Δ	Complexity Δ
...a/rx/internal/operators/OperatorThrottleFirst.java	100% <100%> (ø)	2 <ø> (ø)	❌
...ain/java/rx/internal/operators/OnSubscribeAmb.java	79.13% <ø> (-5.04%)	13% <ø> (ø)
...va/rx/internal/schedulers/EventLoopsScheduler.java	84.84% <ø> (-3.04%)	7% <ø> (ø)
...n/java/rx/subjects/SubjectSubscriptionManager.java	80.71% <ø> (-1.43%)	15% <ø> (-1%)
...n/java/rx/internal/operators/CachedObservable.java	84.23% <ø> (+0.54%)	6% <ø> (ø)	❌
...main/java/rx/internal/operators/OperatorMerge.java	87.06% <ø> (+0.69%)	7% <ø> (ø)	❌
...ava/rx/internal/util/BackpressureDrainManager.java	67.9% <ø> (+1.23%)	21% <ø> (+1%)	✅
...rx/internal/operators/OnSubscribeFromIterable.java	93.9% <ø> (+2.43%)	5% <ø> (ø)	❌
.../rx/internal/schedulers/CachedThreadScheduler.java	90.29% <ø> (+2.91%)	6% <ø> (ø)	❌
...ain/java/rx/internal/schedulers/SchedulerWhen.java	87.83% <ø> (+4.05%)	4% <ø> (ø)	❌
... and 2 more

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