MEDIUM: htx: Try to take a connection over if it has no owner.

MEDIUM: htx: Try to take a connection over if it has no owner.

In the mux detach function, when using HTX, take the connection over if
it no longer has an owner (ie because the session that was the owner left).
It is done for legacy code in proto_http.c, but not for HTX.
Also when using HTX, in H2, try to add the connection back to idle_conns if
it was not already (ie we used to use all the available streams, and we're
freeing one). That too was done in proto_http.c.