Long outage when changing private key

[Derrick Lyndon Pallas]

Thanks for taking a look, I should have spent 30 more minutes 
investigating myself. Please see the patch set I just submitted. 
Resetting the handshake timer is also necessary or else it takes until 
the expiration of that timer to actually happen in my setup. It seemed 
worth putting into a utility function in peers.c, rather than all in 
netdev.c, so I did that. I checked the preshared key case and that seems 
to work fine still. ~Derrick