Enable multiple routing attempts = multi-hop raiden routes

[kilrau]

How it is

Currently raiden changes preimages for payments when one route fails and the next is tried. That's breaking swaps.

To be done:

1. Raiden multi-hop channels raiden#15
2. enable and test multi-hop raiden trades in xud

[kilrau]

understand why raiden's implementation behaves that way

That's clear by now: because of the "refund payments" raiden uses instead of cancelling HTLCs. For the refund payment the preimage ahs to be revealed and thus can't be reused in the second routing attempt. Waiting for the raiden team to come back to us on that.