readBatch(), readAllAvailable() or getSyncReader()

[tyoshino]

Sub issue for #320.

It's still under discussion whether we should take care of the promises overhead. It's discussed at #320. This sub issue makes sense only if the conclusion for the issue turns out to be that we need sync mode.

As @wanderview analyzed in #320 (comment), there should not any big performance difference between them.

But as I questioned in #320 (comment), readBatch() and getSyncReader() have different power for recognition of the consumer's signal. This comment by me elaborates the first bullet point in the comment by Ben.

Domenic +1-ed readAllAvailable() at #320 (comment) to avoid confusion.

So, currently I see 3 approaches:

1. provide readAllAvailable() on async reader
2. provide readBatch(N) on async reader
3. add getSyncReader()

[domenic]

Resolved in #320.