lnrpc: increase max message size for ws proxy

[Roasbeef]

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:

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)

[guggero]

LGTM 🎉