Remove usages of rxscala and replace with Java based code.

[markusthoemmes]

Description

RxScala has been abandoned: ReactiveX/RxScala#244. To unblock the move to Scala 2.13, this removes all usages of it and replaces it with Java interoperable code.

Related issue and scope

Ref #4741

Checklist:

• I signed an Apache CLA.
• I reviewed the style guides and followed the recommendations (Travis CI will check :).

[codecov-io]

Codecov Report

Merging #4744 into master will increase coverage by 35.92%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master    #4744       +/-   ##
===========================================
+ Coverage   42.38%   78.31%   +35.92%     
===========================================
  Files         191      198        +7     
  Lines        8564     8830      +266     
  Branches      609      623       +14     
===========================================
+ Hits         3630     6915     +3285     
+ Misses       4934     1915     -3019

Impacted Files	Coverage Δ
...core/database/cosmosdb/RxObservableImplicits.scala	0% <0%> (ø)	⬆️
...hisk/core/monitoring/metrics/OpenWhiskEvents.scala	94.11% <0%> (ø)
...penwhisk/core/monitoring/metrics/MetricNames.scala	100% <0%> (ø)
...k/core/monitoring/metrics/PrometheusRecorder.scala	81.41% <0%> (ø)
...pache/openwhisk/core/monitoring/metrics/Main.scala	0% <0%> (ø)
...nwhisk/core/monitoring/metrics/KamonRecorder.scala	83.63% <0%> (ø)
.../core/monitoring/metrics/PrometheusEventsApi.scala	90% <0%> (ø)
...nwhisk/core/monitoring/metrics/EventConsumer.scala	87.27% <0%> (ø)
...openwhisk/common/tracing/OpenTracingProvider.scala	21.15% <0%> (+1.92%)	⬆️
...re/database/MultipleReadersSingleWriterCache.scala	98% <0%> (+2%)	⬆️
... and 134 more

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[chetanmeh]

May be simplify it further with SAM. Also using P instead of T as Intellij warned of shadowing here

def toHandler[P](f: (P) => Unit): Action1[P] = (t: P) => f(t)

[markusthoemmes]

Good point! You can tell my scala is a little on the rusty side 😂

[chetanmeh]

Good to get rid of a third party dependency which is not maintained. Thanks @markusthoemmes !

[markusthoemmes]

@chetanmeh told me to merge myself so here we go!

[sinwe]

Why not using https://github.com/reactor/reactor-scala-extensions?

[markusthoemmes]

@sinwe Not sure if that matches up with our usage of reactivex here. I mostly wanted to get rid of the one dependency I removed here without changing the code too much, so this was the quickest way.

[sinwe]

@markusthoemmes if your usage of rxscala is not that much, it's not worth to add additional dependencies. However if you use rxscala extensively and you want to keep track as it progress, e.g: scala 2.13, etc, then it's better you invest some time to replace the dependency completely with another one. There are many libraries similar to rxscala, e.g: Monix, Reactor, etc.

[markusthoemmes]

The code here is purely adaptor code for the CosmosDB lib though, so we have no choice other than using what they use.
OpenWhisk itself uses plain Futures and various akka libs.

[sinwe]

Then you've done the right thing :)