Un-deprecate [SG]et{Item,Byte}sProcessed, re-implement as custom counters. (#676)

As discussed with @dominichamon and @dbabokin, sugar is nice.
Well, maybe not for the health, but it's sweet.
Alright, enough puns.

A special care needs to be applied not to break csv reporter. UGH.
We end up shedding some code over this.
We no longer specially pretty-print them, they are printed just like the rest of custom counters.

Fixes #627.