Multiprocessing sometimes freezes get_geodata

[tgrandje]

This is something I have already encountered on a linux machine. It seems to be linked to the multiprocessing freeze support not being properly handled, but the user might not even know of it without heavy debugging.

It occured to me again while debugging this issue and this is definitely not easy peasy...

As multiprocessing does not seem to be of any value here (compared to multithreading), I propose to switch to pebble ThreadPool which has a close enough API.