Pathfing slow with many relations

[bonndan]

As the number of possible paths in large graphs with many relations grows exponentially, the rendering of svg becomes really slow.

Try to fix by dropping alternatives and/or using more efficient loops.

[bonndan]

b316438 improves the situation a lot, mainly because DEBUG message are silent

@sl0815 this should reduce memory usage since GC should be easier
@Matthimatiker maybe you could have a look if we can squeeze out some more time