net/http: add Transport.MaxConnsPerHost knob

net/http: add Transport.MaxConnsPerHost knob

Add field to http.Transport which limits connections per host,
including dial-in-progress, in-use and idle (keep-alive) connections.

For HTTP/2, this field only controls the number of dials in progress.

Fixes #13957

Change-Id: I7a5e045b4d4793c6b5b1a7191e1342cd7df78e6c
Reviewed-on: https://go-review.googlesource.com/71272
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>