MEDIUM: peers: Use true states for the learn state of a peer

MEDIUM: peers: Use true states for the learn state of a peer

Some flags were used to define the learn state of a peer. It was a bit
confusing, especially because the learn state of a peer is manipulated from
the peer applet but also from the sync task. It is harder to understand the
transitions if it is based on flags than if it is based a dedicated state
based on an enum. It is the purpose of this patch.

Now, we can define the following rules regarding this learn state:

  * A peer is assigned to learn by the sync task
  * The learn state is then changed by the peer itself to notify the
    learning is in progress and when it is finished.
  * Finally, when the peer finished to learn, the sync task must acknowledge
    it by unassigning the peer.