Add agent for handling key revision histories

[tiller-tolbus]

Per resolution to #2

Add a new agent %pki-store to the %token desk that gives us a more complete understanding of the key histories of ships that transact on the %token network.

%pki-store initiates (by command, not ++on-init) by adding a watchdog to %eth-watcher and retrieving a full Ethereum transaction history. It then parses the transaction history into a data structure that can be queried as follows:

• Request: [@p [life=@ud rift=@ud]
• Result: pass

%pki-store then subscribes to %azimuth and updates its state based on each new item in Azimuth's log.

[tiller-tolbus]

%pki-store is now in working order, and %token can subscribe to it.

af78d2a

We thankfully do not need to add a watchdog to %eth-watcher or anything like that, as historical data is only pertinent after the chain launches. By running %pki-store on at least one node, from the initial launch date, we will have a valid historical public key database that can be used to verify transactions.

Now, sharing these across full nodes and making such sharing trustworthy and verifiable is a problem unto itself. But as we can now verify signatures, using a model of signature collection that we can rely on in production, this issue is solved.