Bridgestrap should log the whole pipeline of request/ask-tor/respond

Periodically we have bugs in the bridgestrap subsystem where maybe it isn't measuring bridges as we expect, and it's unclear in these cases if it is a bridgestrap bug or a tor bug or something in between or what.

To help bring order to this situation, it would be great if bridgestrap would log its view of the pipeline:

• i have just received these questions to answer
• i have asked tor to test these bridges
• this is the result i just got from tor
• now i'm sending back these answers

The goal is that I can understand what bridgestrap thinks it is doing, and compare that to what Tor says it is doing, to understand if there are mismatches.

As one edge case, the fact that bridgestrap caches answers (#2) can make it unclear which questions actually got sent to Tor and which didn't.