buildlet: try a bit harder to cancel slow HTTP requests
buildlet: try a bit harder to cancel slow HTTP requests

Use the new Go 1.5 mechanism to abort HTTP requests with a channel.
This is respected by the Go 1.5 http.Transport, which we always use.

This CL shouldn't be necessary (CL 14700 was the real bug), but
doesn't hurt. This still would've probably prevented most of
golang/go#12666

Change-Id: I6890e016ee04183fc0d600baed8046c2f79113d8
Reviewed-on: https://go-review.googlesource.com/14701
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,proppy/build,golang/build,golang/build,proppy/build,golang/build,proppy/build,proppy/build