overflows.py: speed up overflow analysis by fetching all data in one call

I've cut down the runtime of the overflows plugin by ~60% by making only a single call to data.fetch for all channels at the beginning. It's still the slowest plugin, but there's a lot of work in creating all those segment lists to see overflows, so I don't anticipate cutting this down further without some more clever manipulations.

Closes #20 (closed).