Dev trace option to locate non optimal queries

[thihara]

Virtual relations can result in N+1 operations, the developer will not realize that an additional DB operation occurs when accessing a field.

N+1 is not the only issue, it could also result in singular queries that are not necessary.

Add a dev option to print warnings whenever we are loading the virtual relations, so it would be easier to trace non optimal and unintentional queries.

[shaib]

For anybody who wishes to tackle this:
When a previously-deferred field is fetched from DB, it is done in bdmodels.models.BrokenDownModel.refresh_from_db(). At that point, the exact field is known, so it is an excellent point to introduce such a trace.