send local response: clarify call sequence

[kyessenov]

Per discussion in envoyproxy/envoy#36809

[PiotrSikora]

This is the opposite of the intended behavior (which is that proxy_send_local_response() is always a final call).

We want plugins to do something like this:

proxy_on_request_headers() {
   ...
   return proxy_send_local_response(...);
}

and not something like this:

proxy_on_request_headers() {
   ...
   proxy_send_local_response(...);
   do_something_very_important();
   return Pause;
}

The reason this is deferred in Envoy is because the response callbacks (i.e. proxy_on_response_headers() and friends) are called immediately upon sending local response, which leads to re-entrancy problems, but that's another issue (I don't think that plugins should "receive" the local responses they generated themselves).

[kyessenov]

@mpwarres I think we have to specify it one way or another. How does WaaS behave here?