Add command-line option to `globalexport.py` to control the batch size during exports
The global export script is using the import_from_postgis() function from exposure-lib. This function allows to specify the batch size of tiles being processed in one query an it is set to 500 as default. However, our experience tells us that using smaller numbers (e.g. 50) helps avoiding the database turning to sequential scans instead of index scans. Therefore, we need to add a command-line option to globalexport.py to control the batch size.