Coprocessors: Support aggregate functions

Chatting with jgray and holstad at the kitchen table about counts, sums, and other aggregating facility, facility generally where you want to calculate some meta info on your table, it seems like it wouldn't be too hard making a filter type that could run a function server-side and return the result ONLY of the aggregation or whatever.

For example, say you just want to count rows, currently you scan, server returns all data to client and count is done by client counting up row keys. A bunch of time and resources have been wasted returning data that we're not interested in. With this new filter type, the counting would be done server-side and then it would make up a new result that was the count only (kinda like mysql when you ask it to count, it returns a 'table' with a count column whose value is count of rows). We could have it so the count was just done per region and return that. Or we could maybe make a small change in scanner too so that it aggregated the per-region counts.