Update to Morphia 2.0

[lotzk]

In the settings-service we subtype the Setting class to allow for various kinds of settings. Since Morphia, which is used to persist such settings, can't find all subtypes of a specific type, we had to maintain a list of all knowns subtypes and manually go over the according collections to merge the output.

Morphia in version 2.0 does now support querying a whole subtree of a class hierarchy (see MorphiaOrg/morphia#22) and we should update to that version once it is out of beta.