2.x: option to fail for using blockingX on the computation scheduler

[akarnokd]

This PR adds an RxJavaPlugins option failOnNonBlockingScheduler that triggers an IllegalStateException when the user tries to run a blocking method while the execution is on the computation() or single() Scheduler:

Flowable.just(1)
.subscribeOn(Schedulers.computation())
.map(v -> Flowable.just("query").subscribeOn(Schedulers.io()).blockingFirst())
.doOnNext(v -> someAPI(v).subscribeOn(Schedulers.newThread()).blockingSubscribe());
.blockingFirst();

It is an optional setting, default off.

The check is done before going into an await method (and a few other types of blocking). Most blocking operators usually poll the status and try to avoid the actual blocking thus this shouldn't affect synchronous sequences that one extracts a value from.

Detection of a blocking-sensitive scheduler's thread is done by checking the current thread's class for implementing the NonBlockingThread marker interface (currently internal).

The RxThreadFactory has been updated to allow picking a default Thread implementation or a custom one for the newThread(). Note that since #5002 you can create custom schedulers by providing a ThreadFactory.

This works for RxJava's default schedulers but not for AndroidSchedulers.mainThread() where similar blocking should be avoided as well. For them, a plugin-callback action would be more suitable. Question is how that callback should behave (throw, return false, should it be always executed or only when the flag is true).

My proposed solution is to have a plugin callback RxJavaPlugins.setOnBeforeBlocking(BooleanSupplier) that Android users can define the callback for:

RxJavaPlugins.setOnBeforeBlocking(() -> Looper.myLooper() == Looper.getMainLooper())
RxJavaPlugins.setFailOnNonBlockingScheduler(true);

This callback is only executed if the failOnNonBlockingScheduler is set to true.

[codecov-io]

Current coverage is 95.67% (diff: 92.45%)

Merging #5020 into 2.x will increase coverage by 0.14%

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[JakeWharton]

Do you have a use case in mind for exposing this API in addition to just the boolean?

[akarnokd]

See the updated PR description.