websocket: Allow parsing the HTTP version from responses

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Respect the HTTP 204 header when parsing the response

204 meens no content will be sent, so treat the request as only
having headers.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Implement keep alives for http-stream1 channels

We keep one connection around for 10 seconds in case another request
comes for the same thing.

In addition callers can specify a "connection" identifier which
allows them to reuse a specific connection if it is still open. This
is useful for strange connection based authentication schemes.

Closes #1579
Fixes #1570
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>