MINOR: quic: remove <mux_state> field

MINOR: quic: remove <mux_state> field

This patch removes <mux_state> field from quic_conn structure. The
purpose of this field was to indicate if MUX layer above quic_conn is
not yet initialized, active, or already released.

It became tedious to properly set it as initialization order of the
various quic_conn/conn/MUX layers now differ between the frontend and
backend sides, and also depending if 0-RTT is used or not. Recently, a
new change introduced in connect_server() will allow to initialize QUIC
MUX earlier if ALPN is cached on the server structure. This had another
level of complexity.

Thus, this patch removes <mux_state> field completely. Instead, a new
flag QUIC_FL_CONN_XPRT_CLOSED is defined. It is set at a single place
only on close XPRT callback invokation. It can be mixed with the new
utility functions qc_wait_for_conn()/qc_is_conn_ready() to determine the
status of conn/MUX layers now without an extra quic_conn field.