Implement consensus and descriptor stats as materialized views

Kevin has been working on implementing statistics based on network status consensuses and server descriptors as materialized views in the database. These statistics include number of relays, relay versions, relay platforms, and advertised bandwidth. The motivation is to have the database as single interface between metrics-db and metrics-web.

So far, we have Java and huge temporary files to generate consensus and descriptor stats. The database schema changes that Kevin implemented will maintain tables with the required statistics data. Updates to these tables will be triggered after importing new data every hour. Querying the tables will be very efficient.

As discussed on #tor-dev today, we are going to merge Kevin's code into metrics-db soon. This ticket will be used to keep all discussion about merging the code in a single place.