LibIMAP: Wait for a full IMAP response before parsing

We're now sniffing the incoming data to verify the server has sent a
full response, instead of passing partial data to our IMAP parser.
Our parser really can't handle partial input very well, so for the time
being, this heuristic does a much better job of verifying we have full
response before parsing.
It doesn't yet handle unprompted untagged reponses, nor the
"continuation request" responses that start with a `+`, but we don't
fully support those yet.

Userland/Libraries/LibIMAP/Client.h

@@ -63,6 +63,7 @@ private:
     void setup_callbacks();
 
     ErrorOr<void> on_ready_to_receive();
+    bool verify_response_is_complete();
 
     ErrorOr<void> handle_parsed_response(ParseStatus&& parse_status);
     ErrorOr<void> send_next_command();