Review the way tasks are handled

There are lots of pieces of code (quick sort, new join marking, joins, etc.) where there are non-blocking tasks which kick off thread pools to do the processing.

It would be worth revisiting i) to see if we can use a common thread pool (and possibly use intel thread building blocks), and ii) simplify the current structure for blocking threadpools as well.

Something for design discussion.

jakesmith richardkchapman