lnwire: add SingleFundingSignComplete msg to workflow
lnwire: add SingleFundingSignComplete msg to workflow

This commit adds the SingleFundingSignComplete message to the single
funder transaction workflow. This marks the second to last message sent
in the workflow. The message transports Bob’s signature for the
commitment transaction, allowing Alice to broadcast the funding
transaction as she can now refund her inputs.

ddustin/lnd,BitfuryLightning/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,LightningNetwork/lnd,bryanvu/lnd,ddustin/lnd,bryanvu/lnd,BitfuryLightning/lnd