export_results: add --limit option to limit number of builds exported

This parameter will limit the number of builds exported when --url is a top-level job URL. This is useful for speeding up 'rebuilding' of results, since the results app can/does limit many jobs to fewer than the number that might be available for exporting in Jenkins. When number to export > number allowed in database, the app does unnecessary inserts/deletes as it adds builds only to delete them once the threshold is exceeded.