fix #895 Safer scan of ACTUAL and PARENT

[simonbasle]

This commit makes scanning of ACTUAL and PARENT safer by introducing a
safe converter in the Attr class. By default, no converter is defined
and the old behavior of force-casting is still used.

For the two attributes above however, the value from scanUnsafe goes
through Scannable#from first when calling scan or scanOrDefaul.
That way, even though a lot of operators blindly return something that
is Scannable in most case but not necessarily (e.g. a Publisher),
the outer scan will not fail.

Added companion RawAttr for ACTUAL and PARENT which are Attr<Object>
always returning the raw value from the operator (downcasted to Object).

[smaldini]

I wonder if we can just mask RawAttr and hide/remove isConversionSafe. Even tryConvert should be an internal concern for the attributes we manage wdyt ?

[codecov-io]

Codecov Report

Merging #896 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #896      +/-   ##
============================================
+ Coverage     83.63%   83.66%   +0.03%     
- Complexity     3311     3313       +2     
============================================
  Files           323      323              
  Lines         26162    26171       +9     
  Branches       4854     4857       +3     
============================================
+ Hits          21881    21897      +16     
+ Misses         2821     2820       -1     
+ Partials       1460     1454       -6

Impacted Files	Coverage Δ	Complexity Δ
...tor-core/src/main/java/reactor/core/Scannable.java	94.87% <100%> (+0.66%)	18 <4> (ø)	⬇️
...ava/reactor/core/publisher/WorkQueueProcessor.java	69.49% <0%> (-1.7%)	19% <0%> (ø)
...c/main/java/reactor/core/publisher/FluxCreate.java	86.51% <0%> (+0.25%)	8% <0%> (ø)	⬇️
...rc/main/java/reactor/core/publisher/Operators.java	72.51% <0%> (+0.36%)	84% <0%> (ø)	⬇️
.../main/java/reactor/core/publisher/FluxFlatMap.java	94.77% <0%> (+0.4%)	13% <0%> (ø)	⬇️
...ava/reactor/core/publisher/EventLoopProcessor.java	78.68% <0%> (+1.09%)	50% <0%> (+2%)	⬆️
...va/reactor/core/publisher/ParallelMergeReduce.java	70.24% <0%> (+1.65%)	3% <0%> (ø)	⬇️
.../java/reactor/core/publisher/FluxLimitRequest.java	98% <0%> (+4%)	5% <0%> (ø)	⬇️

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[simonbasle]

@smaldini yes, for now we can hide away these concerns, updated