2.x: upgrade Reactive-Streams dependency to 1.0.1

[akarnokd]

This PR upgrades the dependency to Reactive-Streams 1.0.1 final, and

• organizes the other dependency versions into nice variables,
• allows the javadoc generator to link to the RS javadoc.

A couple of remarks on the other dependency versions:

• JMH 1.16: last version that supports Java 6
• TestNG 6.9.10: last version that doesn't break the RS TCK and works with the TestNG Eclipse plugin
• Mockito 2.1.0: I guess upgrading within the minor version has little advertising value.
• JUnit 4.12: Latest version everything supports as of now.

[codecov]

Codecov Report

Merging #5546 into 2.x will increase coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #5546      +/-   ##
===========================================
+ Coverage     96.1%   96.18%   +0.08%     
  Complexity    5822     5822              
===========================================
  Files          631      631              
  Lines        41414    41414              
  Branches      5740     5740              
===========================================
+ Hits         39799    39833      +34     
+ Misses         656      627      -29     
+ Partials       959      954       -5

Impacted Files	Coverage Δ	Complexity Δ
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	95.48% <0%> (-2.26%)	6% <0%> (ø)
...java/io/reactivex/processors/PublishProcessor.java	98.29% <0%> (-1.71%)	45% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	98.3% <0%> (-1.7%)	11% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...reactivex/internal/queue/SpscLinkedArrayQueue.java	96.82% <0%> (-0.8%)	38% <0%> (-1%)
...operators/flowable/FlowableWithLatestFromMany.java	99.24% <0%> (-0.76%)	7% <0%> (ø)
...ternal/operators/observable/ObservableGroupBy.java	96.36% <0%> (-0.61%)	2% <0%> (ø)
...nternal/operators/observable/ObservableReplay.java	98.05% <0%> (-0.56%)	23% <0%> (ø)
... and 16 more

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