Allow alternative settlement via direct blockchain payment

[holisticode]

Currently on threshold crossing, cheques are being issued automatically by the debitor.

It should be allowed that settlement would be able to be initiated via direct blockchain payment.

[Eknir]

This should only be allowed, in my opinion, if the counterparty agrees with this. It touches many points of ethersphere/SWIPs#22, where such configuration would be allowed if both nodes agree that this is preferred. @diegomasini

[holisticode]

This has been described by @nagydani in the track sync of today, 23rd September 2019. So this should be discussed with him.