Remove -->inc

Turns out it's not worth the hassle - we're talking milliseconds per
millions of items - so restore use of -->count for general use, especially
when part of a bigger set of aggregates in use.
Improve performance of tally

As tally only has one job, manually perform the grouping and counting. This
is a much better optimisation and gives 6x better performance in CPU and GC
times when testing with millions of items.
Finish removing -->inc