Instead of running the domark() timer on half the MarkInterval option,
we take the FreeBSD approach and run on each TIMERINTVL (30 sec), same
as the doflush() timer. This allows greater resolution on the MARK log
message, meaning we would be off by < 30 seconds instead of the current
MarkInterval / 2.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>