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.
Revert back to .Inc() for most things
Merge branch 'lock-avoidance-and-histograms'