Parallel gpart sort

That's essentially a copy-paste of the part version.

Indices are still of type size_t to be compatible with the rest of the code. We can move to int later on.

Added 2 commits:

• 6afeba8b - More const-correctness
• c2790396 - One missing instance of task_type_psort in engine.c

Reassigned to @pdraper

Looks good to me, I assume you've tested it as well? Re-assigning to @pdraper.

This will presumably go away when we switch to bucket-sort.

Added 61 commits:

• c2790396...31fe964e - 59 commits from branch master
• 8a86e252 - Merge branch 'master' into parallel_gpart_sort
• 7088644a - Post-merge changes

Ok, that's now ready to go with the latest changes to master.

Status changed to merged

mentioned in commit ba115a69