When we verify an HTTP signature,
* If we know the key, check in the DB whether we know the actor lists it. If it
doesn't, and there's room left for keys, HTTP GET the actor and update the DB
accordingly.
* If we know the key but had to update it, do the same, check usage in DB and
update DB if needed
* If we don't know the key, record usage in DB
However,
* If we're GETing a key and discovering it's a shared key, we GET the actor to
verify it lists the key. When we don't know the key at all yet, that's fine
(can be further optimized but it's marginal), but if it's a key we do know,
it means we already know the actor and for now it's enough for us to rely
only on the DB to test usage.
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