feature/conn25: check online when picking connector

So that when control notices a node isn't responding and sends that info
down in a netmap we will stop picking that node as a connector. This
doesn't provide great failover (it takes several minutes) but it's
better than not failing over at all.

Updates tailscale/corp#47250

Signed-off-by: Fran Bull <fran@tailscale.com>