funding: add Memo to InitFundingMsg struct

We add byte-array field called Memo to the InitFundingMsg struct.
We also provide a value for this from
rpcserver.go#parseOpenChannelReq().

funding/manager.go

@@ -291,6 +291,10 @@ type InitFundingMsg struct {
 	// support explicit channel type negotiation.
 	ChannelType *lnwire.ChannelType
 
+	// Memo is any arbitrary information we wish to store locally about the
+	// channel that will be useful to our future selves.
+	Memo []byte
+
 	// Updates is a channel which updates to the opening status of the
 	// channel are sent on.
 	Updates chan *lnrpc.OpenStatusUpdate

rpcserver.go

@@ -2203,6 +2203,7 @@ func (r *rpcServer) parseOpenChannelReq(in *lnrpc.OpenChannelRequest,
 		ChannelType:       channelType,
 		FundUpToMaxAmt:    fundUpToMaxAmt,
 		MinFundAmt:        minFundAmt,
+		Memo:              []byte(in.Memo),
 	}, nil
 }