NewClientConn starts a goroutine (which reads cc.singleUse at start),
so it's not safe to set cc.singleUse after calling NewClientConn.

Change-Id: I5c28db15e8e8fd8ec5ef0e04b7f81e61efbd873e
Reviewed-on: https://go-review.googlesource.com/25230Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>