see #1804 Enforce 100K task cap on Schedulers.boundedElastic()

[simonbasle]

Another late change, but if we introduce the boundedElastic() singleton version of the scheduler, we might as well make it truly bounded, including in the number of task submissions it can "absorb" during a spike. Used to be unbounded, now bounded to 100K tasks.

This is also more likely to uncover problematic scheduling of blocking tasks. Users switching from elastic() to boundedElastic() and seeing RejectedExecutionExceptions should challenge why their Reactor app schedules so many blocking tasks, and if still legitimate and necessary, use a newBoundedElastic instance that is better tuned to their workload.

[codecov-io]

Codecov Report

Merging #1900 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1900      +/-   ##
============================================
+ Coverage     81.54%   81.61%   +0.06%     
- Complexity     3970     3971       +1     
============================================
  Files           373      373              
  Lines         30769    30769              
  Branches       5792     5792              
============================================
+ Hits          25092    25111      +19     
+ Misses         4078     4063      -15     
+ Partials       1599     1595       -4

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/reactor/core/scheduler/Schedulers.java	89.06% <100%> (ø)	89 <0> (-1)	⬇️
.../java/reactor/core/publisher/BlockingIterable.java	78.12% <0%> (-1.05%)	7% <0%> (ø)
...eactor/core/publisher/ParallelMergeSequential.java	80.31% <0%> (-1.04%)	7% <0%> (ø)
...java/reactor/core/scheduler/ExecutorScheduler.java	79.16% <0%> (-0.7%)	12% <0%> (ø)
...ain/java/reactor/core/publisher/FluxConcatMap.java	90% <0%> (+0.27%)	7% <0%> (ø)	⬇️
...c/main/java/reactor/core/publisher/FluxReplay.java	84.29% <0%> (+0.45%)	28% <0%> (+1%)	⬆️
...eactor/core/scheduler/BoundedElasticScheduler.java	83.72% <0%> (+0.58%)	39% <0%> (ø)	⬇️
.../java/reactor/core/scheduler/ElasticScheduler.java	85.84% <0%> (+1.76%)	26% <0%> (ø)	⬇️
...actor/core/publisher/FluxOnBackpressureLatest.java	83.5% <0%> (+3.09%)	2% <0%> (ø)	⬇️
...in/java/reactor/core/publisher/FluxWindowWhen.java	85.57% <0%> (+5.76%)	2% <0%> (ø)	⬇️

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