This makes sure we advertise xroutes after they are retracted.  They
will be flushed by the normal GC mechanism.

This allows sending more than 70 or so xroutes for a single gateway.
Let's hope nobody actually uses this feature.

For now, we don't keep track of forwarded requests, so we might still
reply with the old seqno.

This leaves 6 bits in hello messages for upward-compatible changes.