MEDIUM: peers: Add infos in peer structure to know if it is synchronized or not

MEDIUM: peers: Add infos in peer structure to know if it is synchronized or not

Info about the last update message sent are now saved for each peer. The
shared-table and the update message id are saved. These information are used
when a ack message is received to know if it matches the last update message
sent. When this matches, we are sure the peer as received all updates sent
and is synchronized. This information is saved thanks to the flag
PEER_F_SYNCHED.

So, at any time, we know if a peer is synchronized or not.