BUG/MEDIUM: mux-quic: adjust buggy proxy closing support

BUG/MEDIUM: mux-quic: adjust buggy proxy closing support

The wake handler detects if the frontend is closed. This can happen if
the proxy has been disabled individually or even on process soft-stop.
Before this patch, in this condition QCS instances were freed before
being detached from the cs_endpoint. This clearly violates the haproxy
connection architecture and cause a BUG_ON statement crash in cs_free().

To handle this properly, cs_endpoint is notified by setting RD_SH|WR_SH
on connection flags. The cs_endpoint will thus use the detach operation
which allows the QCS instance to be freed.

This code allows the soft-stop process to complete as soon as possible.
However, the client is not notified about the connection closing. It
should be done by emitting a H3 GOAWAY + CONNECTION_CLOSE. Sadly, this
is impossible at this stage because the listener sockets are closed so
the quic-conn cannot use it to emit new frames. At this stage the client
will most probably detect connection closing on its idle timeout
expiration.

Thus, to completely support proxy closing/soft-stop, important
architecture changes are required in QUIC socket management. This is
also linked with the reload feature.