ws: Add CockpitWebResponse:done signal

Fires when the web response is done outputting the response to
the IO stream and is ready to go away.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Implement support for HTTP keep alives

This makes us much faster, especially when loading over high
latency links or using SSL.

We make it the responsibility of CockpitWebServer to close the
connections, and we check for further requests on a connection.

CockpitWebResponse indicates whether it thinks a given connection
will be reusable after it has sent a response on it.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>