Per-Replica Encryption Keys

Spinning off discussion from GothenburgBitFactory/taskchampion-sync-server#3.

@djmitche:

> Also note that encryption keys are per-client, not per-replica. That is, my laptop, desktop, and phone all share the same encryption key. We should have a process to handle a compromised key, but I think it would involve migrating all replicas to a new client_id (and new encryption key).

@ryneeverett:

> This doesn't strike me as a great security story. Maybe it's daunting but would it be worth considering per-replica keys? I would argue that most people don't roll keys but they do roll devices.

> I recall at least one person reporting that their team uses bugwarrior to aggregate their issues into a single synchronized taskwarrior database. I believe the entire team would be one "client" under the new model, so when somebody leaves the company everybody on the team would need to roll the key.

@djmitche:

> Let's split off the issue of key rotation. I don't know of a simple mechanism for each replica to have a different key but still be able to exchange information with other replicas.