mailbench: Count individual messages in batch mode
mailbench: Count individual messages in batch mode

Take advantage of the closed control loop between mailbench and
mail-enqueue to count each message as it's delivered.  This also means
we can use much larger batch sizes without skewing the counts.

aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6,aclements/sv6,aclements/sv6