When the monotonic timer is initialized to 0 right after the system is
booted the daemon responded with COOKIES for COOKIE_CALMDOWN_DELAY (10s).
Since the COOKIE verification code actually produces an overflow for
COOKIE_LIFETIME (10s) it wouldn't even accept properly returned COOKIEs.
Checking for last_cookie makes sense anyway as that condition must only
apply if we actually sent a COOKIE before.