net/http: add Transport.IdleConnTimeout

net/http: add Transport.IdleConnTimeout

Don't keep idle HTTP client connections open forever. Add a new knob,
Transport.IdleConnTimeout, and make the default be 90 seconds. I
figure 90 seconds is more than a minute, and less than infinite, and I
figure enough code has things waking up once a minute polling APIs.

This also removes the Transport's idleCount field which was unused and
redundant with the size of the idleLRU map (which was actually used).

Change-Id: Ibb698a9a9a26f28e00a20fe7ed23f4afb20c2322
Reviewed-on: https://go-review.googlesource.com/22670
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>