tweak log sanitizing regex so that it doesn't hide datastore keys (sources, responses)
poll task: use a transaction when updating the source at the end

fixes the last writer race where Source.last_webmention_sent wasn't getting set: a poll task would insert a propagate task, it would run and set last_webmention_sent, and *then* the poll task would finish and rewrite source with the old value.

R=@kylewm. hey kyle, mind looking this over quickly? it's a little delicate, so i'd love another pair of eyes on it.

(you're also always welcome to comment on any other changes you see fly by, of course!)
update testutil to handle activitystreams Source.base_object() change