Roundtrip signature verification

[tiller-tolbus]

Now that we have a %pki-store full of public keys, it should be possible to verify signatures on pooled transactions.

[tiller-tolbus]

35d6ffc

Using %pki-store on a fakezod or comet presents a problem. Kicking the roundtrip verification using Azimuth keys until later and calling this done on a naive signature level.