MINOR: peers: Add flags to report the peer state to the resync task

MINOR: peers: Add flags to report the peer state to the resync task

As the previous patch, this patch is also part of the refactoring of peer
locking mechanisme. Here we add flags to represent a transitional state for
a peer. It will be the resync task responsibility to update the peers state
accordingly.

A peer may be in 4 transitional states:

  * accepted : a connection was accepted from a peer
  * connected: a connection to a peer was established
  * release  : a peer session was released
  * renewed  : a peer session was released because it was replaced by a new
               one. Concretly, this will be equivalent to released+accepted

If none of these flags is set, it means the transition, if any, was
processed by the resync task, or no transition happened.