MINOR: peers: Reorder and slightly rename PEER flags

MINOR: peers: Reorder and slightly rename PEER flags

There are too many holes in peer flags. So let's reorder them. In addition,
PEER_F_RESYNC_REQUESTED flag was renamed to PEER_F_DBG_RESYNC_REQUESTED to
clearly state it is a flag set for debugging purpose.

Finally, PEER_TEACH_RESET was replaced by PEER_TEACH_FLAGS and the bitwise
complement operator is now used on lines updating the peer flags. It is a
far more common way to do (in HAProxy code at least) and less surprising.