Reuse a single transaction for all INSERT statements for a single
upload. This cuts about 50% of the runtime off an invocation of
the tests with -cloud, and about 25% of the runtime off an /upload run
on App Engine.

Change-Id: I90068f60420cfb67ae693f18ed83b341c2e483bd
Reviewed-on: https://go-review.googlesource.com/35069Reviewed-by: Russ Cox <rsc@golang.org>