OperatorDoEach could do with some work

[davidmoten]

While looking for emissions after terminal events I was looking at OperatorDoEach which is called by doOnNext, doOnError, doOnCompleted, and doOnEach of course. I noticed these things:

• lots of wasted allocations per call (use of ActionSubscriber when all that is needed is an ActionObserver) and passing three actions to OperatorDoEach when for three of the four operators we only need one action.
• it doesn't call RxJavaHooks.error for secondary error emissions from upstream
• it does call Exceptions.throwIfFatal for all error emissions (do we want to do this generally? I haven't seen it much outside error catching on externally supplied functions or actions)

I'd quite like to see doOnNext and doOnError in particular more efficient because I use them a lot. Should I create dedicated Operator/OnSubscribe classes for the single action operators or perhaps one shared one that has an extra parameter (to indicate what notification type is being used)?

[akarnokd]

You may optimize it into an OnSubscribe. I'm not sure about splitting as it triples the method count for the sake of saving 16 bytes.

[davidmoten]

I could also use ActionObserver instead of ActionSubscriber I suppose (saves 32 bytes). Let me know what you think of the throwIfFatal issue and I'll do it.

[akarnokd]

The throwIfFatal shouldn't be there; the operators should not call onError with a fatal exceptions anymore but throw them immediately - this should be also reviewed and fixed in 2.x as well.

[akarnokd]

FYI, I follow this list when I port tests.

[akarnokd]

Closing via #4262.