Add local error consumer hooks support

[smaldini]

Related to #435

• contextualize Operators.onErrorDropped and onNextDropped
• remove unused Operators.onErrorDropped(error, supp)
• add few missing Operators.onNextDropped
• contextualize Operators.onOperatorsError
• contextualize Operators.onRejectedExecution

[codecov-io]

Codecov Report

Merging #830 into master will decrease coverage by 0.02%.
The diff coverage is 57.04%.

@@             Coverage Diff              @@
##             master     #830      +/-   ##
============================================
- Coverage     83.07%   83.04%   -0.03%     
- Complexity     3172     3184      +12     
============================================
  Files           316      316              
  Lines         25526    25677     +151     
  Branches       4771     4774       +3     
============================================
+ Hits          21206    21324     +118     
- Misses         2864     2898      +34     
+ Partials       1456     1455       -1

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/reactor/core/publisher/MonoDoFinally.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...ore/src/main/java/reactor/core/publisher/Mono.java	95.07% <ø> (ø)	228 <0> (ø)	⬇️
...ore/src/main/java/reactor/core/publisher/Flux.java	100% <ø> (ø)	466 <0> (ø)	⬇️
.../reactor/core/publisher/FluxDoFinallyFuseable.java	100% <ø> (ø)	2 <0> (ø)	⬇️
.../reactor/core/publisher/MonoDoFinallyFuseable.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...in/java/reactor/core/publisher/FluxWindowWhen.java	74.33% <0%> (-0.34%)	2 <0> (ø)
...main/java/reactor/core/publisher/FluxInterval.java	76.19% <0%> (-1.86%)	2 <0> (ø)
...ava/reactor/core/publisher/FluxWindowBoundary.java	80.57% <0%> (ø)	2 <0> (ø)	⬇️
.../java/reactor/core/publisher/StrictSubscriber.java	59.7% <0%> (ø)	15 <0> (ø)	⬇️
.../reactor/core/publisher/FluxDelaySubscription.java	75.86% <0%> (ø)	3 <0> (ø)	⬇️
... and 159 more

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[simonbasle]

@smaldini I opened #844 for the follow-ups, focusing on calling Operators hooks with a Context and looking into that Context for local hooks for now.