Prevent windowWhen from retaining windows

[simonbasle]

see #975

@smaldini as I wanted to use Disposable.Composite, I came across the need for a mean of applying an operation to each Disposable.
This in turn brought up the need for the Composite to have a generic type, so the forEach Consumer could meaningfully work with the data.

On top of that, in a second commit, I used the newly augmented Composite to manage windows and clean a window when it expires.

wdyt?

[codecov-io]

Codecov Report

Merging #984 into master will decrease coverage by 0.02%.
The diff coverage is 52.17%.

@@             Coverage Diff              @@
##             master     #984      +/-   ##
============================================
- Coverage     83.67%   83.65%   -0.03%     
- Complexity     3374     3376       +2     
============================================
  Files           327      327              
  Lines         26631    26656      +25     
  Branches       4917     4922       +5     
============================================
+ Hits          22283    22298      +15     
- Misses         2879     2882       +3     
- Partials       1469     1476       +7

Impacted Files	Coverage Δ	Complexity Δ
...r-core/src/main/java/reactor/core/Disposables.java	86.47% <50%> (-3.62%)	20 <1> (ø)
...actor/core/scheduler/EmptyCompositeDisposable.java	12.5% <50%> (-1.79%)	1 <1> (ø)
...or-core/src/main/java/reactor/core/Disposable.java	88.88% <50%> (-11.12%)	1 <0> (ø)
...in/java/reactor/core/publisher/FluxWindowWhen.java	74.44% <56.25%> (+0.11%)	2 <0> (ø)	⬇️
.../java/reactor/core/publisher/FluxLimitRequest.java	94% <0%> (-4%)	5% <0%> (ø)
...ain/java/reactor/core/publisher/MonoPublishOn.java	88.33% <0%> (-3.34%)	2% <0%> (ø)
...va/reactor/core/publisher/ParallelMergeReduce.java	68.59% <0%> (-3.31%)	3% <0%> (ø)
...rc/main/java/reactor/core/publisher/FluxRange.java	83.33% <0%> (-1.34%)	5% <0%> (ø)
.../java/reactor/core/publisher/UnicastProcessor.java	86.51% <0%> (-0.57%)	65% <0%> (-1%)
...ava/reactor/core/publisher/WorkQueueProcessor.java	70.76% <0%> (-0.43%)	19% <0%> (ø)
... and 8 more

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[simonbasle]

superseded by #1013