Reconsider the query log format

The current way in which we handle query logs has several issues:
it's racy, files can get pretty large, the search algorithm is
too complex, and we can only do basic filtering and very little
aggregation. But we also don't have a lot
of alternatives. Ideally we would like to have something
like an SQL database, à la SQLite, but that
would require cgo, which comes with its own bag
of issues. Package
modernc.org/sqlite
seems interesting, but it's a work-in-progress and currently
doesn't support all our platforms.

I will update the issue when we'll think of something
better. Perhaps SQLite is not completely
out of the question if we find a way to cross-compile it
without too much pain for the developers.