MEDIUM: sink: start applets asynchronously

MEDIUM: sink: start applets asynchronously

Since d9c1d33fa1 ("MEDIUM: applet: Add support for async appctx startup
on a thread subset"), it is now possible to delay appctx's init: for that
it is required that the .init callback is defined on the applet.

When the applet will be processed on the first run, applet API will
automatically finish the applet initialization. Thus we explicitly
call appctx_wakeup() on the applet to schedule it for initial run
instead of calling appctx_init() ourselves.

This is done in prevision of the next patch in order to be able to
schedule the applet on a different thread from the one executing
sink_forward_session_create() function.

Note: 'out_free_appctx' label was removed since it is no longer used.