MINOR: session: do not release conn in session_check_idle_conn()

MINOR: session: do not release conn in session_check_idle_conn()

session_check_idle_conn() is called to flag a connection already
inserted in a session list as idle. If the session limit on the number
of idle connections (max-session-srv-conns) is exceeded, the connection
is removed from the session list.

In addition to the connection removal, session_check_idle_conn()
directly calls MUX destroy callback on the connection. This means the
connection is freed by the function itself and should not be used by the
caller anymore.

This is not practical when an alternative connection closure method
should be used, such as a graceful shutdown with QUIC. As such, remove
MUX destroy invokation : this is now the responsability of the caller to
either close or release immediately the connection.