The HTTP client had a limit for the maximum number of idle connections
per-host, but not a global limit.

This CLs adds a global idle connection limit too,
Transport.MaxIdleConns.

All idle conns are now also stored in a doubly-linked list. When there
are too many, the oldest one is closed.

Fixes #15461

Change-Id: I72abbc28d140c73cf50f278fa70088b45ae0deef
Reviewed-on: https://go-review.googlesource.com/22655Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>