Removes DrupalUrlAlias  #47 and optimizes maps index

Finishes #47.

This PR removes DrupalUrlAlias. I tried a couple of things, but the most robust way turned out to be to simply moving the url_alias.dst column into the node table, thereby making dst lookups faster and easier and reducing the amount joins one has to do. It also slimmed down the code a bit here and there.

A second, smaller part involves a minor optimization of the maps index that tangents off #33. It involves optimizing the view rendering time and in my dev environment it went from ~800ms to ~130ms.

Because this involves a central table, I'd advise checking this code in development mode using a copy of the production database to see if there are any issues. As far as I was able to check, things look to be working correctly :).