* layer primitives: connections, requests, responses, and streams.
 *
 * This class supports [asynchronous canceling][cancel]. This is intended to have the smallest
 * blast radius possible. If an HTTP/2 stream is active, canceling will cancel that stream but not
 * the other streams sharing its connection. But if the TLS handshake is still in progress then
 * canceling may break the entire connection.
 */