This is the chunked half of https://golang.org/cl/49570044 .

We want full reads to return EOF as early as possible, when we
know we're at the end, so http.Transport client connections are eagerly
re-used in the common case, even if no Read or Close follows.

To do this, make the chunkedReader.Read fill up its argument p []byte
buffer as much as possible, as long as that doesn't involve doing
any more blocking reads to read chunk headers. That means if we
have a chunk EOF ("0\r\n") sitting in the incoming bufio.Reader,
we see it and set EOF on our final Read.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/58240043