When SetSubnetRoutes is called, also send updatePeerChanged to all
other connected nodes so they re-fetch their MapResponse and learn
about the updated AllowedIPs. Without this, peers never see new
subnet routes until they happen to reconnect to the control server.
Discovered while working on a three-VM natlab subnet router
integration test, coming later.
Updates #13038
Change-Id: I20e7a2fda994a8ab0e7a24240e6eae536f4f5f15
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
ccef06b968