Try to improve rrl timing

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.