[MINOR] stream_interface: make use of an applet descriptor for IO handlers

[MINOR] stream_interface: make use of an applet descriptor for IO handlers

I/O handlers are still delicate to manipulate. They have no type, they're
just raw functions which have no knowledge of themselves. Let's have them
declared as applets once for all. That way we can have multiple applets
share the same handler functions and we can store their names there. When
we later need to add more parameters (eg: usage stats), we'll be able to
do so in the applets themselves.

The CLI functions has been prefixed with "cli" instead of "stats" as it's
clearly what is going on there.

The applet descriptor in the stream interface should get all the applet
specific data (st0, ...) but this will be done in the next patch so that
we don't pollute this one too much.