Make it fully buildable on JDK 8 (including the StackWalker usage)

[bsideup]

No description provided.

[codecov-io]

Codecov Report

Merging #1483 into master will increase coverage by 0.03%.
The diff coverage is 80.95%.

@@             Coverage Diff              @@
##             master    #1483      +/-   ##
============================================
+ Coverage     84.11%   84.14%   +0.03%     
- Complexity     3897     3898       +1     
============================================
  Files           359      359              
  Lines         29790    29831      +41     
  Branches       5522     5531       +9     
============================================
+ Hits          25057    25101      +44     
+ Misses         3101     3095       -6     
- Partials       1632     1635       +3

Impacted Files	Coverage Δ	Complexity Δ
...e/src/main/java/reactor/core/publisher/Traces.java	70.62% <80.95%> (+3.96%)	25 <0> (ø)	⬇️
...va/reactor/core/publisher/ParallelMergeReduce.java	68.59% <0%> (-1.66%)	3% <0%> (ø)
...ava/reactor/core/publisher/WorkQueueProcessor.java	69.42% <0%> (-0.42%)	19% <0%> (ø)
...java/reactor/core/publisher/FluxSwitchOnFirst.java	73.98% <0%> (-0.34%)	3% <0%> (ø)
...c/main/java/reactor/core/publisher/FluxReplay.java	84.16% <0%> (-0.32%)	25% <0%> (ø)
...eactor/core/publisher/ParallelMergeSequential.java	80.31% <0%> (+0.51%)	7% <0%> (ø)	⬇️
...ain/java/reactor/core/publisher/FluxConcatMap.java	90.42% <0%> (+0.56%)	7% <0%> (ø)	⬇️
...java/reactor/core/scheduler/ExecutorScheduler.java	79.86% <0%> (+0.69%)	12% <0%> (ø)	⬇️
...c/main/java/reactor/core/publisher/FluxCreate.java	86.63% <0%> (+0.99%)	8% <0%> (ø)	⬇️
.../java/reactor/core/publisher/BlockingIterable.java	78.12% <0%> (+1.04%)	7% <0%> (ø)	⬇️
... and 2 more

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