Performance improvements

[raphaelm]

Fixes three performance issues on large databases:

• We did not have an index on OrderPosition.server_id, which made typical lookups quite slow. We should have indexes on all server_id fields, they are cheap and prevent future problems.
• We did not have indexes on all foreign keys used for joins. This is generally a best practice, databases like PostgreSQL would automatically create them. So let's create them! I see some speedups especially on Quota_Item table and the likes of it.
• We had an index on Receipt(canceled) and ReceiptLine(canceled). This was a bad idea. There are only two possible values of this field (true / false) that usually have a ratio somewhere between 50:50 and 80:20. This means that utilizing the index to scan the table for all (non-)canceled lines only reduces the work by half. However, since SQLite's optimizer isn't too clever, it prevented using actually useful indexes. On a query like SELECT SUM(price) AS sum FROM ReceiptLine WHERE receipt = ? AND canceled = 0, SQLite would do a scan over the canceled = 0 index and then filter out tens of thosands of wrong receipts, instead of just scanning over the receipt = ? index and filter out 5 of 10 lines.