channeldb: persist channel Memo field and read it when fetching

We add a Memo field to the OpenChannel DB struct. We also persist
it using a tlv record. We then pass the Memo value from the
InitFundingReserveMsg when creating a new reservation for the channel.
Finally, we also read Memo field when fetching channel from DB.

lnwallet/reservation.go

@@ -416,6 +416,7 @@ func NewChannelReservation(capacity, localFundingAmt btcutil.Amount,
 			Db:                   wallet.Cfg.Database,
 			InitialLocalBalance:  ourBalance,
 			InitialRemoteBalance: theirBalance,
+			Memo:                 req.Memo,
 		},
 		pushMSat:      req.PushMSat,
 		pendingChanID: req.PendingChanID,