MEDIUM: peers: Use true states for the peer applets as seen from outside

MEDIUM: peers: Use true states for the peer applets as seen from outside

This patch is a cleanup of the recent change about the relation between a
peer and the applet used to deal with I/O. Three flags was introduced to
reflect the peer applet state as seen from outside (from the sync task in
fact). Using flags instead of true states was in fact a bad idea. This work
but it is confusing. Especially because it was mixed with LEARN and TEACH
peer flags.

So, now, to make it clearer, we are now using a dedicated state for this
purpose. From the outside, the peer may be in one of the following state
with respects of its applet:

 * the peer has no applet, it is stopped (PEER_APP_ST_STOPPED).

 * the peer applet was created with a validated connection from the protocol
   perspective. But the sync task must synchronized it with the peers
   section. It is in starting state (PEER_APP_ST_STARTING).

 * The starting starting was acknowledged by the sync task, the peer applet
   can start to process messages. It is in running state
   (PEER_APP_ST_RUNNING).

 * The last peer applet was released and the associated connection
   closed. But the sync task must synchronized it with the peers section. It
   is in stopping state (PEER_APP_ST_STOPPING).

Functionnaly speaking, there is no true change here. But it should be easier
to understand now.

In addition to these changes, __process_peer_state() function was renamed
sync_peer_app_state().