This introduces a new attribute that allows behavior to keep
the TCP connection(s) to a HTTP1/HTTP2 host longer or shorter
than the default of 120 seconds.
Note that the server might still close the connection earlier.
Fixes: QTBUG-20726
Fixes: QTBUG-91440
Change-Id: I7da64230a78c642c12c0ddbe6b678cf17c3aafde
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>