* Apply inbound settings and send an acknowledgement to the peer that provided them.
     *
     * We need to apply the settings and ack them atomically. This is because some HTTP/2
     * implementations (nghttp2) forbid peers from taking advantage of settings before they have
     * acknowledged! In particular, we shouldn't send frames that assume a new `initialWindowSize`
     * until we send the frame that acknowledges this new size.
     *