Loosen precision of timestamp in initiation message

Right now the timestamp field of the initiation message is too precise due (`TimeStamper::stamp`). It should be truncated to avoid acting as a time oracle which could aid in other attacks.

The Linux kernel implementation choses to truncate the value based on the possible number of initiations per second, see https://github.com/WireGuard/WireGuard/commit/c62836ef83bf.