MEDIUM: peers: Save date of the last update to wake the peer applet

MEDIUM: peers: Save date of the last update to wake the peer applet

Instead of looking for new updates in each updates lists to wake a peer
applet up, we now only detect that some updates should have been inserted by
comparing the date of the last update inserted in the list and the last
update sent to the peer.

It is not 100% accurrate of course. Some extra wakeups may be observed. But
this should not lead to any spinning loop because the operation is performed
by the sync task. This task is woken up when a timeout is fired or when an
update was inserted. However, this saves several loops on the updates lists.