[REORG] session: move the data_ctx struct to the stream interface's applet

This is in fact where those parts belong to. The old data_state was replaced
by applet.state and is now initialized when the applet is registered. It's
worth noting that the applet does not need to know the session nor the
buffer anymore since everything is brought by the stream interface.

It is possible that having a separate applet struct would simplify the
code but that's not a big deal.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index 50e1f9c..d462308 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -302,6 +302,7 @@
  * it is solicited. The task's processing function must call the applet's
  * function before returning. It must be deleted by the task handler using
  * stream_int_unregister_handler(), possibly from within the function itself.
+ * It also pre-initializes applet.state to zero.
  */
 struct task *stream_int_register_handler(struct stream_interface *si, struct si_applet *app)
 {
@@ -315,6 +316,7 @@
 	si->connect = NULL;
 	si->target.type = TARG_TYPE_APPLET;
 	si->target.ptr.a = app;
+	si->applet.state = 0;
 	si->release   = NULL;
 	si->flags |= SI_FL_WAIT_DATA;
 	return si->owner;