BUG/MEDIUM: peers: recent applet changes broke peers updates scheduling

BUG/MEDIUM: peers: recent applet changes broke peers updates scheduling

Since appctx are scheduled out of streams, it's pointless to wake up
the task managing the stream to push updates, they won't be seen. In
fact unit tests work because silent sessions are restarted after 5s of
idle and the exchange is correctly scheduled during startup!

So we need to notify the appctx instead. For this we add a pointer to
the appctx in the peer session.

No backport is needed of course.