MEDIUM: conn-stream/applet: Add a data callback for applets

MEDIUM: conn-stream/applet: Add a data callback for applets

data callbacks were only used for streams attached to a connection and
for health-checks. However there is a callback used by task_run_applet. So,
si_applet_wake_cb() is first renamed to cs_applet_process() and it is
defined as the data callback for streams attached to an applet. This way,
this part now manipulates a conn-stream instead of a stream-interface. In
addition, applets are no longer handled as an exception for this part.