Add logic to replace the standard RxJava 2 schedulers with reactor ones

[akarnokd]

This PR allows the user to replace the standard RxJava 2 schedulers with its Reactor-Core counterparts (and restore the defaults as well) so that redundant worker threads could be avoided.

[bsideup]

Hi @akarnokd,

Nice to see this topic being discussed :)

Have you seen reactor/reactor-core#1655 by any chance?
Would love to hear your opinion on it!

[akarnokd]

I don't know what that issue is about.

[bsideup]

We identified a problem when users shade Reactor (aka relocate it with different package name) that the schedulers are obviously not shared anymore. It correlates with your motivation ( "so that redundant worker threads could be avoided" ) in this PR, but from a different perspective, this is why I thought you may be interested in joining the conversation there.
One way of solving it was a suggestion to use the common ForkJoinPool provided by the JDK.
There is also an open question whether we should use it by default in Schedulers.parallel.

[simonbasle]

Thanks for the PR @akarnokd 🙇

Regarding the core issue quoted by @bsideup, have you got any insight on shading? Has it come up in RxJava 2 world that some library would shade RxJava (with package relocation) and then duplicate worker threads issue would arise for downstream users that use both said library and vanilla RxJava?

[akarnokd]

You'd have to know where RxJavaPlugins ended up and do the same hooking as with this PR.