net/http: fix Transport race(s) with high GOMAXPROCS

net/http: fix Transport race(s) with high GOMAXPROCS

Also adds a new test for GOMAXPROCS=16 explicitly, which now passes
reliably in a stress loop like:

$ go test -c
$ (while ./http.test -test.v -test.run=Concurrency; do echo pass; done ) 2>&1 | tee foo; less foo

(It used to fail very quickly and reliably on at least Linux/amd64)

Fixes #3793

R=golang-dev, adg, r
CC=golang-dev
https://golang.org/cl/6347061