BUG/MINOR: mux_quic: fix BE conn removal on app shutdown

BUG/MINOR: mux_quic: fix BE conn removal on app shutdown

When QUIC application layer is shut for a backend connection, the
connection is immediately removed from its idle pool. This is a nice
optimization as this prevents a future streams to try to reuse an
unusable connection. This is implemented since the following commit.

  00d668549e46b34d29ea3daa1f6dd42b5251a365
  MINOR: mux-quic: do not reuse connection if app already shut

However, this removal is not correctly performed as it is used
conn_delete_from_tree(). For private connections, this can cause crashes
as they are stored in the session instead. Thus, connection status is
now properly check, and alternatively session_unown_conn() is used if
stored in the session.

This must be backported up to 3.3.