CLEANUP: peers: use less confusing state/status code names

CLEANUP: peers: use less confusing state/status code names

Currently, all states, all status codes and a few constants used in
the peers are all prefixed with "PEER_SESSION_". It's confusing because
there is no way to know which one is a state, a status code or anything
else. Thus, let's rename them this way :

    PEER_SESS_ST_* : states
    PEER_SESS_SC_* : status codes

Additionally the states have been numbered from zero and contigously.
This will allow us not to have to deal with the stream interface
initialization anymore and to ease debugging using enums.