Add Fees to internal Routing

[Dominik1999]

Abstract

Transfers using internal routing don't work at the moment if at least one mediator has set a fee >0. We need to add fees to the transfer.

An outline of what we want:

1. We add for every transaction that uses internal routing a fee amount, e.g. 2% per hop or 10% in general
2. We ensure that the transfer initiator does not reveal the secret if the intended amount to be sent did not reach the target (because of greedy mediators in the middle)
3. We add a fee_default_value to the Raiden Settings to ensure that no transfer is being initiated with higher fees than that value