packet: Implement rekeying based on the recommendation from RFC's
The default rekeying recommendations are specified in
RFC4344 Section 3 (First and Second Rekeying Recommendations).
Additionally, the rekeying can be specified in configuration
file/options allowing us to turn the rekeying off, base it
on time or make it more strict.
The code is highly inspired by the OpenSSH rekeying code.
Signed-off-by: Jakub Jelen <email@example.com>
Reviewed-by: Daiki Ueno <firstname.lastname@example.org>
Reviewed-by: Andreas Schneider <email@example.com>