Provide onDiscard support in UnicastProcessor

[OlegDokuka]

This PR ensures that when UnicastProcessor is canceled all the elements are safely discarded so no leaks are going to appear

Signed-off-by: Oleh Dokuka shadowgun@i.ua

[codecov-io]

Codecov Report

Merging #2102 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #2102      +/-   ##
============================================
+ Coverage     81.96%   82.02%   +0.05%     
  Complexity     4091     4091              
============================================
  Files           380      380              
  Lines         31247    31246       -1     
  Branches       5800     5799       -1     
============================================
+ Hits          25612    25628      +16     
+ Misses         4061     4052       -9     
+ Partials       1574     1566       -8     

Impacted Files	Coverage Δ	Complexity Δ
.../java/reactor/core/publisher/UnicastProcessor.java	87.07% <100.00%> (-0.08%)	67.00 <1.00> (-1.00)
...eactor/core/publisher/ParallelMergeSequential.java	84.97% <0.00%> (-0.52%)	7.00% <0.00%> (ø%)
.../main/java/reactor/core/publisher/FluxFlatMap.java	94.95% <0.00%> (-0.19%)	19.00% <0.00%> (ø%)
...c/main/java/reactor/core/publisher/FluxReplay.java	84.83% <0.00%> (+0.14%)	29.00% <0.00%> (ø%)
...ain/java/reactor/core/publisher/FluxConcatMap.java	90.55% <0.00%> (+0.27%)	7.00% <0.00%> (ø%)
...rc/main/java/reactor/core/publisher/Operators.java	81.09% <0.00%> (+0.52%)	136.00% <0.00%> (ø%)
...c/main/java/reactor/core/publisher/MonoCreate.java	74.24% <0.00%> (+0.75%)	3.00% <0.00%> (ø%)
...eactor/core/publisher/FluxSubscribeOnCallable.java	73.33% <0.00%> (+0.83%)	3.00% <0.00%> (ø%)
.../java/reactor/util/concurrent/MpscLinkedQueue.java	89.47% <0.00%> (+1.31%)	18.00% <0.00%> (+1.00%)
...ava/reactor/core/publisher/FluxWindowBoundary.java	81.60% <0.00%> (+1.72%)	2.00% <0.00%> (ø%)
... and 2 more

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[simonbasle]

@OlegDokuka this one should actually be backported in 3.2.x 😨