INITIAL
sent initial heartbeart,  | 
got back an epoch         |
                          V
                        ACTIVE -------------------------> SHUTDOWN
lease expired or revoked  |  ^                          |    ^
(stale metadata,          |  | lease restored               | controller granted
 id conflict,             |  | with new broker epoch        | controlled shutdown
 unable to communicate,   V  |                              |
 etc.)                  FENCED -----------------------------+