BUG/MEDIUM: peer: Notify the applet won't consume data when it waits for sync

BUG/MEDIUM: peer: Notify the applet won't consume data when it waits for sync

When the peer applet is waiting for a synchronisation with the global sync
task, we must notify it won't consume data. Otherwise, if some data are
already waiting in the input buffer, the applet will be woken up in loop and
this wil trigger the watchdog. Once synchronized, the applet is woken up. In
that case, the peer applet must indicate it is going to consume data again.

This patch should fix the issue #2656. It must be backported to 3.0.