Histogram of delivery size, and batch sizes; lock avoidance

 - serveDrain acquires a lot of locks for counters, which has now been
   reduced to 1 per counter per batch
 - We see elevated parse times for, presumably large batches, but have
   no measurements in place to capture batch sizes to debug. Add
   histograms for these two things.
I was wrong about this as Counter.Inc() uses sync.atomic, which doesn't take a lock. So undo the Inc stuff, except for the linesCounter and I'll merge.