lnrpc: increase max message size for ws proxy

In this commit, we increase the max message size for the ws proxy. We
have a similar setting for the normal gRPC server which was tuned to be
able to support decoding `GetNetworkInfo` as the channel graph got
larger. We keep the default buffer size of 64 KB, but allow that to be
expanded to up to 4 MB (current value) to decode larger messages.

One alternative would be to modify the `Split` function to break up
larger lines into smaller ones. We'd need to double check that the
libraries at a higher level of abstraction can handle the chunks. The
scan function would look something like:
```go
splitFunc := func(data []byte, eof bool) (int, []byte, error) {
        if len(data) >= chunkSize {
                return chunkSize, data[:chunkSize], nil
        }

        return bufio.ScanLines(data, eof))
}
scanner.Split(splitFunc)
```

lnrpc/websocket_proxy.go

@@ -38,6 +38,11 @@ const (
 	// an arbitrary non-empty message that has no deeper meaning but should
 	// be sent back by the client in the pong message.
 	PingContent = "are you there?"
+
+	// MaxWsMsgSize is the largest websockets message we'll attempt to
+	// decode in the gRPC <-> WS proxy. gRPC has a similar setting used
+	// elsewhere.
+	MaxWsMsgSize = 4 * 1024 * 1024
 )
 
 var (
@@ -413,9 +418,18 @@ func (r *requestForwardingReader) CloseWriter() {
 // what's written to it and presents it through a bufio.Scanner interface.
 func newResponseForwardingWriter() *responseForwardingWriter {
 	r, w := io.Pipe()
+
+	scanner := bufio.NewScanner(r)
+
+	// We pass in a custom buffer for the bufio scanner to use. We'll keep
+	// with a normal 64KB buffer, but allow a larger max message size,
+	// which may cause buffer expansion when needed.
+	buf := make([]byte, 0, bufio.MaxScanTokenSize)
+	scanner.Buffer(buf, MaxWsMsgSize)
+
 	return &responseForwardingWriter{
 		Writer:  w,
-		Scanner: bufio.NewScanner(r),
+		Scanner: scanner,
 		pipeR:   r,
 		pipeW:   w,
 		header:  http.Header{},