net/http: don't re-use Transport connections if we've seen an EOF
This the second part of making persistent HTTPS connections to
certain servers (notably Amazon) robust.
See the story in part 1: https://golang.org/cl/
76400046/
This is the http Transport change that notes whether our
net.Conn.Read has ever seen an EOF. If it has, then we use
that as an additional signal to not re-use that connection (in
addition to the HTTP response headers)
Fixes #3514
LGTM=rsc
R=agl, rsc
CC=golang-codereviews
https://golang.org/cl/
79240044