Replace routes.PrimaryRoutes reads with NodeStore. Connect bumps
SessionEpoch; Disconnect re-checks it inside UpdateNode so the
check and mutation are atomic against a concurrent Connect on
the same node.
The connect_race regression test is carried in its final
SessionEpoch form.
Updates #3203
Existing HA tests verify server-side primary election; these add
end-to-end assertions from a viewer client's perspective, that marking
the primary unhealthy or revoking its approved route propagates through
the netmap so the viewer sees the flip.
Updates #3157