Try to improve rrl timing
Add a +burst option to mdig so that we have a second to setup the mdig calls then they run at the start of the next second.
RRL uses 'queries in a second' as a approximation to 'queries per second'. Getting the bursts of traffic to all happen in the same second should prevent false negatives in the system test.
We now have a second to setup the traffic in. Then the traffic should be sent at the start of the next second. If that still fails we should move to +burst=<now+2> (further extend mdig) instead of the implicit <now+1> as the trigger second.
(cherry picked from commit 92cdc7b6)