rpcserver: fetch balance for ChannelBalance from disk

This commit modifies the ChannelBalance RPC to fetch the balance from
disk since channels are now able to efficiently retrieved from disk due
to recent index that have been added. Previously this RPC would only
return accurate information if we had an active connection to the
counter party for each channel.

rpcserver.go

@@ -479,11 +479,14 @@ func (r *rpcServer) WalletBalance(ctx context.Context,
 func (r *rpcServer) ChannelBalance(ctx context.Context,
 	in *lnrpc.ChannelBalanceRequest) (*lnrpc.ChannelBalanceResponse, error) {
 
+	channels, err := r.server.chanDB.FetchAllChannels()
+	if err != nil {
+		return nil, err
+	}
+
 	var balance btcutil.Amount
-	for _, peer := range r.server.Peers() {
-		for _, snapshot := range peer.ChannelSnapshots() {
-			balance += snapshot.LocalBalance
-		}
+	for _, channel := range channels {
+		balance += channel.OurBalance
 	}
 
 	return &lnrpc.ChannelBalanceResponse{Balance: int64(balance)}, nil