Rewrite node keys to point to primary PMS.

Among other things, this should allow some queued videos to work that are
geoblocked (somewhere other than the US), since the lookup will occur at the
user's PMS. It's also necessary for certain services that require user auth.

Included in this change is a hidden pref to set the primary server machine ID.
At some point we'll probably want to expose this, but most users only have a
single (owned) server anyway, and nobody has ever complained before.