Move reactor-tools into the core repository

[bsideup]

Imported from https://github.com/reactor/reactor-tools

(I did not touch the implementation to make the review easier, will follow up with a few cleanups)

[simonbasle]

lgtm for a move of the module from its own repo to the core repo 👍

[codecov-io]

Codecov Report

Merging #1731 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1731      +/-   ##
============================================
+ Coverage     81.87%   81.93%   +0.05%     
+ Complexity     3801     3799       -2     
============================================
  Files           362      362              
  Lines         30047    30120      +73     
  Branches       5593     5632      +39     
============================================
+ Hits          24602    24678      +76     
+ Misses         3901     3898       -3     
  Partials       1544     1544

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/reactor/core/scheduler/SchedulerTask.java	68.42% <0%> (-5.27%)	12% <0%> (-1%)
...rc/main/java/reactor/core/publisher/Operators.java	80.33% <0%> (-0.15%)	125% <0%> (-1%)
...in/java/reactor/core/publisher/FluxBufferWhen.java	92.08% <0%> (+0.35%)	2% <0%> (ø)	⬇️
.../main/java/reactor/core/publisher/FluxFlatMap.java	94.78% <0%> (+0.38%)	13% <0%> (ø)	⬇️
...c/main/java/reactor/core/publisher/FluxCreate.java	83.37% <0%> (+0.48%)	8% <0%> (ø)	⬇️
...c/main/java/reactor/core/publisher/FluxReplay.java	85.77% <0%> (+1.45%)	25% <0%> (ø)	⬇️
.../java/reactor/core/scheduler/ElasticScheduler.java	85.24% <0%> (+1.63%)	26% <0%> (ø)	⬇️

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[smaldini]

LGTM we will work on the improvements themselves post M2