Duplicate key constraint when multiple requests try to populate same item

[mentos1386]

When doing same request at the same time, with empty database. One of them will fail with duplicate key constraint. As both of them try to populate entety from esi to db. We should handle key constraint and try to fetch entity from database again (as now it is populated and we don't have to).

[mentos1386]

When #126 is done, we should reevaluate if this is even going to happen, as we don't populate entities the same way anymore.