MINOR: mux-quic: do not reuse connection if app already shut

MINOR: mux-quic: do not reuse connection if app already shut

QUIC connection graceful closure is performed in two steps. First, the
application layer is closed. In the context of HTTP/3, this is done with
a GOAWAY frame emission, which forbids opening of new streams. Then the
whole connection is terminated via CONNECTION_CLOSE which is the final
emitted frame.

This commit ensures that when app layer is shut for a backend
connection, this connection is removed from either idle or avail server
tree. The objective is to prevent stream layer to try to reuse a
connection if no new stream can be attached on it.

New BUG_ON checks are inserted in qmux_strm_attach() and h3_attach() to
ensure that this assertion is always true.