cmd/lncli: increase lightning gRPC block size to 50MB

In this commit, we increase the gRPC block size from 4MB to 50MB.
Recently, the output of `lncli describegraph` has hit the block size cap
due to the expansion of the mainnet graph. Without this attempts to
fetch the graph returns an error of:
```
[lncli] rpc error: code = ResourceExhausted desc = grpc: received message larger than max (4246753 vs. 4194304)
```

With this commit, we give ourselves some breathing room. It's important
to note that the max message size limit is a _client side_ setting. As a
result, any developers driving `lnd` with gRPC will also need to raise
their block size limit as well if they wish to fetch the graph over
gRPC.

cmd/lncli/main.go

@@ -37,6 +37,10 @@ const (
 var (
 	defaultLndDir      = btcutil.AppDataDir("lnd", false)
 	defaultTLSCertPath = filepath.Join(defaultLndDir, defaultTLSCertFilename)
+
+	// maxMsgRecvSize is the largest message our client will receive. We
+	// set this to ~50Mb atm.
+	maxMsgRecvSize = grpc.MaxCallRecvMsgSize(1 * 1024 * 1024 * 50)
 )
 
 func fatal(err error) {
@@ -131,11 +135,10 @@ func getClientConn(ctx *cli.Context, skipMacaroons bool) *grpc.ClientConn {
 
 	// We need to use a custom dialer so we can also connect to unix sockets
 	// and not just TCP addresses.
-	opts = append(
-		opts, grpc.WithDialer(
-			lncfg.ClientAddressDialer(defaultRPCPort),
-		),
-	)
+	genericDialer := lncfg.ClientAddressDialer(defaultRPCPort)
+	opts = append(opts, grpc.WithDialer(genericDialer))
+	opts = append(opts, grpc.WithDefaultCallOptions(maxMsgRecvSize))
+
 	conn, err := grpc.Dial(ctx.GlobalString("rpcserver"), opts...)
 	if err != nil {
 		fatal(fmt.Errorf("unable to connect to RPC server: %v", err))