Improvements to Salt implemented for converting Ridges to relANNIS 4

These are some improvements to Salt I made to better support relANNIS conversion:

- don't create threads in GraphTraverserModule traverse() function
- use iterator when iterating over all outgoing edges instead of having a counter variable and using get(idx) on the list
- make sure SDocumentStructureRootAccessor#getSRootsByRelation() returns a list without duplicates
- removed empty finalize() function in SDocumentGraphImpl: every class that implements finalize can't be deleted directly by the GC but will be put in a background thread that executes the finalize method in some point of the future