Add new PKG protocol

[davidlazar]

When adding a new PKG we might want to do something like: temporarily stop all registrations at the existing PKGs, have each existing PKG send a signed copy of its users database to the new PKG, and then have the new PKG pre-populate its users database by merging the existing databases before re-enabling registrations. To merge databases, the new PKG should only accept a user record if the all the other PKGs agree on it; otherwise, it should ask the user to register (and verify their email address).