MINOR: xprt_qstrm: implement close callback

MINOR: xprt_qstrm: implement close callback

Implement close callback for xprt_qstrm layer. This is called when a
connection is prematurely closed following a connect failure. Its
purpose is to clean up all xprt resources.

A special care is required for the frontend side. Indeed,
xprt_qstrm_io_cb() can call close callback via conn_create_mux() on the
latter failure. The tasklet should then immediately be stopped as the
whole xprt layer has been freed as well.