Optimise store indexing

[Mirko-von-Leipzig]

Our sql queries are (likely) missing a bunch of indices.

The main way this will show up is (eventually) with slow queries once the db is populated. While #609 may help identify these bottlenecks, one can already identify most candidates using explain query plan.

Queries should in general be using indices (search) and avoiding full row scans. Missing indices can therefore be identified, added and rechecked to ensure they're being used.

A more robust solution would involve running this in tests somehow, and ensuring there are no scan involved. There may of course be exceptions, but these can be whitelisted.

[bobbinth]

Related: #57.