avoid geoJSON loading race condition. speed improvement.

- if geoJSON files specified later in argv loaded too quickly, processing might begin before earlier files had finished loading
- Array.forEach() doesn't break when you return false; .every() does. Failing to realize this meant that every geoJSON boundary was tested against every task, instead of the loop exiting after the first match (by default). This was equivalent to the --slow flag always being set. Matching tasks are a small percentage of overall tasks so this does not produce a dramatic speed increase, but it is the originally intended behavior and should help somewhat.