Attempt an implementation of the relay-search database using MongoDB or CouchDB

Our current relay-search function takes forever to return results. There's legacy/trac#2922 (moved) for improving the database schema to better support searching for single relays. That ticket assumes that we'll continue to use PostgreSQL.

Today I looked into MongoDB for the web server log analysis, and I wonder if MongoDB or CouchDB might be an alternative for implementing the relay-search database.

Every consensus or descriptor could be represented as a document with references to other documents. Indexes could make typical search queries fast. We don't need complicated Map/Reduce functions, because we're only searching and looking up data, not aggregating anything. (That's also the reason why I think this is worth trying out---replacing the metrics database that aggregates statistics with MongoDB/CouchDB may not make as much sense.) Maybe we should run a simple comparison of the new PostgreSQL database that ExoneraTor uses, and that is highly optimized for searches, to an implementation using MongoDB or CouchDB.

I don't know if such a solution will perform better than a PostgreSQL-based solution. I think we should try to find out.