MINOR: applet: replace cs_applet_shut() with appctx_shut()

MINOR: applet: replace cs_applet_shut() with appctx_shut()

The former takes a conn_stream still attached to a valid appctx,
which also complicates the termination of the applet. Instead, let's
pass the appctx which already points to the endpoint, this allows us
to properly detach the conn_stream before the call, which is cleaner
and safer.