This commit stores temporary registration data in cache, instead of
memory allowing us to only have actually registered machines in the
database.

This commit removes the field from the database and does a DB migration
**removing** all unregistered machines from headscale.

This means that from this version, all machines in the database is
considered registered.

This commit removes the two extra caches (oidc, requested time) and uses
the new central registration cache instead. The requested time is
unified into the main machine object and the oidc key is just added to
the same cache, as a string with the state as a key instead of machine
key.