In the brief window between getConn and persistConn.roundTrip,
a cancel could end up going missing.

Fix by making it possible to inspect if a cancel function was cleared
and checking if we were canceled before entering roundTrip.

Fixes #10511

Change-Id: If6513e63fbc2edb703e36d6356ccc95a1dc33144
Reviewed-on: https://go-review.googlesource.com/9181Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>