When the client of a connector assigns transit IP addresses for a
connector we need to let wireguard know that packets for the transit IPs
should be sent to the connector node. We do this by:
* keeping a map of node -> transit IPs we've assigned for it
* setting a callback hook within wireguard reconfig to ask us for these
extra allowed IPs.
* forcing wireguard to do a reconfig after we have assigned new transit
IPs.
And this commit is the last part: forcing the wireguard reconfig after a
new address assignment.
Fixestailscale/corp#38124
Signed-off-by: Fran Bull <fran@tailscale.com>