Use fundchannel_cancel after fundchannel_complete

[ZmnSCPxj]

ElementsProject/lightning#2740 has been implemented. The "correct" way to implement a failure in the fundchannel_complete of some node is to execute fundchannel_cancel in the other nodes that succeeded their fundchannel_completes.

This was closed only a short time ago and it seems the last update to this repository was 23 days ago, so potentially it could be updated now for this new fact.

A precis, is that a fundchannel_cancel may cause the other side to disconnect (especially if older version of C-Lightning), it may be a good thing to connect them again afterwards in case of a failure.