Make submeta attributes multivalued

When indexing a object relation list datatype, eZ Find pushes the meta information for all the related objects in one single solr submeta attribute.
The submeta name definition uses the default *non-multivalued* suffix and makes the indexing fail for the whole object.

This pull request adds the subattribute prefix to the submeta field name.

Aside question: Do we actually need to index the meta information for the related objects? 
I don't see any real-life situation where this information could be used in conjunction with Solr.