MINOR: applet: Save the "use-service" rule in the stream to init a service applet

MINOR: applet: Save the "use-service" rule in the stream to init a service applet

When a service is initialized, the "use-service" rule that was executed is
now saved in the stream, using "current_rule" field, instead of saving it
into the applet context. It is safe to do so becaues this field is unused at
this stage. To avoid any issue, it is reset after the service
initialization. Doing so, it is no longer necessary to save it in the applet
context. It was the last usage of the rule pointer in the applet context.

The init functions for TCP and HTTP lua services were updated accordingly.