Change FetchMode to enhance performance

We discovered heavy performance problems when a single group with (very) many MapLayers is being requested. In our case such a request took about 25 - 120 seconds, which is not acceptable.

The solutions seems to be easy: By changing the FetchMode at the MapLayer-relation from SUBSELECT to JOIN, the same requests will be answered in less than 5 seconds.

It would be great if anyone could review (and merge) this PR!