MEDIUM: stream-int: replace xprt->shutw calls with conn_data_shutw()

MEDIUM: stream-int: replace xprt->shutw calls with conn_data_shutw()

Now that the connection performs the correct controls when shutting down,
use that in the few places where conn->xprt->shutw() was called. The calls
were split between conn_data_shutw() and conn_data_shutw_hard() depending
on the argument. Since the connection flags are updated, we don't need to
call conn_data_stop_send() anymore, instead we just have to call
conn_cond_update_polling().