MEDIUM: applet: Add support for async appctx startup on a thread subset

MEDIUM: applet: Add support for async appctx startup on a thread subset

It is now possible to start an appctx on a thread subset. Some controls were
added here and there. It is forbidden to start a backend appctx on another
thread than the local one. If a frontend appctx is started on another thread
or a thread subset, the applet .init callback function must be defined. This
callback function is responsible to finalize the appctx startup. It can be
performed synchornously. In this case, the appctx is started on the local
thread. It is not really useful but it is valid. Or it can be performed
asynchronously. In this case, .init callback function is called when the
appctx is woken up for the first time. When this happens, the appctx
affinity is set to the current thread to be able to start the session and
the stream.