Fill raw queue from exactly one recv() system call.

        Set self.eof when connection is closed.

def fill_rawq(self):
        """Fill raw queue from exactly one recv() system call.

        Set self.eof when connection is closed.
        """
        if self.irawq >= len(self.rawq):
            self.rawq = b''
            self.irawq = 0
        # The buffer size should be fairly small so as to avoid quadratic
        # behavior in process_rawq() above
        buf = yield from self._reader.read(50)
        self.eof = (not buf)
        self.rawq = self.rawq + buf