BytesStreamOutput default size should be 2k instead of 32k
We changed the default of BytesStreamOutput (used in various places in ES) to 32k from 1k with the assumption that most stream tend to be large. This doesn't hold for example when indexing small documents and adding them using XContentBuilder (which will have a large overhead).

Default the buffer size to 2k now, but be relatively aggressive in expanding the buffer when below 256k (double it), and just use oversize (1/8th) when larger to try and minimize garbage and buffer copies.

relates to #3624
closes #3638