DB import multiprocessing - more functional

[gjn]

...as we do with the tiling. We should use generator functions. Otherwise, with long imports, some processes will stop really working. I've ended up having 2 processes working in the end.

[loicgasser]

That's what I was trying to achieve with my last PR, I am iterating through a GDAL layer to get the features one by one but I am not sure the features are actually streamed from the shapefile. So not 100% sure this is doable with GDAL. But yes we should look into it.