Improve searching for relays in metrics database

Our relay search function on the metrics website has serious performance problems. Some searches return after under a second, but some searches take 2 minutes or longer. It's okay for a search to take a few seconds, but there shouldn't be a variance this high.

All searches are based on a single (very large) table that contains one row per relay listed in a network status consensus. Our current assumption why searches are slow is that indexes have grown too large.

Sebastian and I tried to create separate tables for the fields that users can search for, which looked promising. But after one of the steps to populate these helper tables did not finish after five days, we gave up.

Someone should brainstorm about redesigning our database schema and try out a couple of approaches to search for relays with a couple months of data. Once it turns out that one approach is better than the current one, we also need a migration strategy to convert our database to the new schema.