MAJOR: applet: applet scheduler rework.

MAJOR: applet: applet scheduler rework.

In order to authorize call of appctx_wakeup on running task:
- from within the task handler itself.
- in futur, from another thread.

The appctx is considered paused as default after running the handler.

The handler should explicitly call appctx_wakeup to be re-called.

When the appctx_free is called on a running handler. The real
free is postponed at the end of the handler process.