Use database for storing GetTor (and other) stats instead of files

We want to migrate all stats files used for plotting graphs to tables in the database, so that we have the database as single interface between metrics-db and metrics-web. Some of the data can be derived from tables that are already in the database (number of relays, relay versions, etc.; see #1844 (moved)). The remaining data needs to be imported manually by metrics-db.

Kevin added this functionality as part of his GSoC project. We are going to merge his code as discussed on #tor-dev today. This ticket will be used for the discussion about merging.