WIP: Feature/avoid starting up if not migrated

This merge request aims to implement a verification to ensure the couch schema has been migrated before starting up soledad server.

It also includes a fix for an ugly bug in which the couch server configuration was loaded from disk for every request.

I think the bits missing to remove WIP from this merge request are:

• check if the solution presented here is good or if there's a better way to implement.
• check if the database and document where the couch schema version is being stored is appropriate.