Move the checking of single valued attributes back into the tdb backend.

Move the checking of single valued attributes back into the tdb backend.

The backend is the only place that can do this properly. It makes no
sense to do it anywhere else. As a result of it moving out of the
backend we ended up with some bugs causing multiple values in single
valued attributes (eg. isDeleted), which can really damage the
inregrity of the database.

For the override of single valued values needed for deleted linked
attributes we should use attribute flags.

This reverts commit 1949864417f3d10fb8996df7db259649eb777271.