The list of supported HTTP methods is available in JS land now so there
is no longer any need to pass a stringified version of the method to the
parser callback, it can look up the method name for itself.
Saves a call to v8::Eternal::Get() in the common case and a costly
v8::String::NewFromOneByte() in the uncommon case.
If a client sends a lot more pipelined requests than we can handle, then
we need to provide backpressure so that the client knows to back off.
Do this by pausing both the stream and the parser itself when the
responses are not being read by the downstream client.
Fix GH-6214
