Wrong usage of longs as part of database keys

[markus1978]

In the index class and index feature indexes, I use longs (extrinsic IDs) as part of the string typed index prefix. This is not ideal because of the alphanumerical order of db indexes. On the other hand it is also irrelevant, because nothing depends on the right order. Even though the numerical successor of key might be alphanumerically larger than a numerical smaller key, we don't have a problem because the prefixes are followed by a unique byte ("_") before the arbitrary rest of the keys.