Feature/jmh+improvements

[OlegDokuka]

This PR provides the following:

• bug fix for the case when the assembly hooks are enabled in Reactor so the cancellation may end up exceptionally with NPE
• provides some fixes in tests
• improves the performance of AbstractStreamObserverAndPublisher by specifying the lowerTild for next prefetch (it means that we don't have to wait for full prefetch fulfillment so upstream have some space to send, and downstream does not call request to frequently)
• provides reference JMH benchmarks
• updates versions of libs

[rmichela]

I think you can combine the two JMH modules into a single module with one runner. You can generate RxJava and Reactor stubs in the same POM and run both harnesses side by side. This will cut out a bunch of code duplication.

[rmichela]

Can you please put bullet points in the PR description for the fixes you made and the dependency upgrades you made?

[rmichela]

Failed tests:

clientToServerBackpressure(com.salesforce.rxgrpc.BackpressureIntegrationTest): expected:<[1]L> but was:<[0]L>
serverToClientBackpressure(com.salesforce.rxgrpc.BackpressureIntegrationTest): expected:<[1]L> but was:<[2]L>

[codecov-io]

Codecov Report

Merging #151 into master will increase coverage by 0.96%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #151      +/-   ##
============================================
+ Coverage      47.2%   48.17%   +0.96%     
+ Complexity      116      115       -1     
============================================
  Files            28       28              
  Lines           841      822      -19     
  Branches         81       81              
============================================
- Hits            397      396       -1     
+ Misses          420      401      -19     
- Partials         24       25       +1

Impacted Files	Coverage Δ	Complexity Δ
...rpc/common/AbstractStreamObserverAndPublisher.java	76.24% <100%> (-0.98%)	49 <3> (-1)
...tivegrpc/common/AbstractSubscriberAndProducer.java	90.65% <100%> (+0.42%)	55 <0> (ø)	⬇️
...n/java/com/salesforce/rxgrpc/stub/ClientCalls.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...n/java/com/salesforce/rxgrpc/stub/ServerCalls.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...a/com/salesforce/reactorgrpc/stub/ServerCalls.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...a/com/salesforce/reactorgrpc/stub/ClientCalls.java	0% <0%> (ø)	0% <0%> (ø)	⬇️
...src/main/java/com/salesforce/rxgrpc/GrpcRetry.java	100% <0%> (+12.9%)	0% <0%> (ø)	⬇️
...ain/java/com/salesforce/reactorgrpc/GrpcRetry.java	100% <0%> (+43.75%)	0% <0%> (ø)	⬇️

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[OlegDokuka]

@rmichela fixed all comments