etcd watch websocket doesn't handle messages longer than 64 KB

[vitalif]

Hi!
There's a problem in grpc-websocket-proxy that affects etcd's websocket watches.
Problem is that it doesn't handle server messages (i.e replies!) longer 64 KB.
The root cause is Golang's bufio.Scanner having a hard-coded maxTokenSize value equal to 64 KB.
Error message in this case is:
time="2020-10-18T15:42:56Z" level=warning msg="scanner err: bufio.Scanner: token too long"
It's easy to reproduce, you just need to create a key/value entry in etcd with a value longer than 64 KB and you won't get it back in the notification.
A simple test example is here: https://gist.github.com/vitalif/a634ac0543e6cacdda4ec288d922d9cf
I've just sent a dirty fix here: tmc/grpc-websocket-proxy#25
I don't know what's the proper way to fix it since it may involve patching Golang standard library so I'm posting it here for your review :)

[vitalif]

Resubmitted a better fix in #12402.