Split resort task

Following on #584, this improves the parallellism and performance of the stars_resort task. It helps with @rgb' infamous step 96.

• Before: http://icc.dur.ac.uk/~jch/tasks/Test1/step96r.html
• After: http://icc.dur.ac.uk/~jlvc76/SWIFT/ResortTask3/step96r.html

Main changes:

• Only flag the cells that have received new stars for resorting.
• Push the resort_task down 2 levels instead of having it at the top.

Also read/write some more globals to/from the check-point files.