AbstractOnSubscribe Batching

[benjchristensen]

How does one performed batched requests when AbstractOnSubscribe invokes onNext every time?

We should be able to generate a backpressure aware Observable that handles function invocation but doesn't prohibit batch performance.

[davidmoten]

Yep, that would be nice.

[akarnokd]

I'll think about it.

[akarnokd]

See #3118 which is likely to replace AbstractOnSubscribe.