Ref T2783. This is basically a more refined version of D10400, which churned a bit on things like SSH key storage, the actual way the signing protocol shook out, etc.
- When Phabricator tries to make an intra-cluster service call as the omnipotent user, sign it with the host's device key.
- Add bin/almanac register to say "this host is X device, identified by private key Y". This stores the keypair locally, adds the public key to Almanac, and trusts it.
Net effect is that once a host has been registered, the daemons can make calls to other nodes as the omnipotent user. This is primarily necessary so they can access repository API methods on remote hosts.