Make changes to bridgedescs module for bulk-processing tarballs

I recently finished re-processing the entire bridge descriptor archive for legacy/trac#19317 (moved). However, I had to make some changes to avoid running out of memory or wasting time on unnecessary operations. I now went through the changes and cleaned them up a bit, because I'd like to merge some/most/all (?) of them for the next time we need to bulk-process the bridge descriptor archive. I'll post a branch once I have a ticket number.

We should discuss which of these commits should go in by default (maybe ed48f03, ae5c53c, and e514d30?), which should only be enabled in a special bulk-processing mode (maybe df96751, 27cbfc8, and 68b29c2?), which should have their own config option (ugh!), or which we drop because we don't need as badly for processing descriptors in bulk.

Clearly, these commits need work, but I figured it's better to clean them up a bit now than attempt to do that in four or eight weeks. Branch follows in a minute.