net: reduce number of memory allocations during IO operations
Embed all data necessary for read/write operations directly into netFD.

benchmark                    old ns/op    new ns/op    delta
BenchmarkTCP4Persistent          27669        23341  -15.64%
BenchmarkTCP4Persistent-2        18173        12558  -30.90%
BenchmarkTCP4Persistent-4        10390         7319  -29.56%

This change will intentionally break all builders to see
how many allocations they do per read/write.
This will be fixed soon afterwards.

R=golang-dev, alex.brainman
CC=golang-dev
https://golang.org/cl/12413043