an idempotent method (e.g. GET), in case the HTTP server
    timed them out.

    @ivar persistent: Boolean indicating whether connections should be
        persistent. Connections are persistent by default.

    @ivar maxPersistentPerHost: The maximum number of cached persistent
        connections for a C{host:port} destination.
    @type maxPersistentPerHost: C{int}

    @ivar cachedConnectionTimeout: Number of seconds a cached persistent
        connection will stay open before disconnecting.

    @ivar retryAutomatically: C{boolean} indicating whether idempotent
        requests should be retried once if no response was received.

    @ivar _factory: The factory used to connect to the proxy.

    @ivar _connections: Map (scheme, host, port) to lists of
        L{HTTP11ClientProtocol} instances.

    @ivar _timeouts: Map L{HTTP11ClientProtocol} instances to a
        C{IDelayedCall} instance of their timeout.

    @since: 12.1
    râ