site: New PROD message

We introduce a new message PROD which requests that the peer initiate
a key exchange with us, if it doesn't already have a key.

This helps significantly reduce a possible "dead period" when one end
of a connection is restarted during key exchange.  The dead period is
now limited to the time taken for the interrupted key exchange to time
out.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>