...one nice thing to include in the docstring would be the expected cost in http fetches and datastore queries for the common cases: 1) source domain_url isn't a webmention target, or 2) a silo post that we've already seen and discovered. i think the answer is 0/0 for 1) (since it's memcached) and 0/1 for 2), right?

i know i'm beating a dead horse; thanks for making that so good already. i expect we'll find at least one or two unexpected case once we push it live that we'll want to optimize, but that's ok.