v0.88.0

• Behavior change: journal registers are a small set of labels which are used
  for cooperative, transactional locking. Where before a set of checked registers
  must always match current journal registers for an append to proceed, now an
  exception exists in the case where a journal has a fully empty set
  of registers (as is the case on recovery if journal consistency is lost).
• gazctl journals reset-head now issues concurrent resets to enumerated journals,
  and no longer fails if an attempt to reset a consistent journal loses an append race.
• Makefiles now build local Docker images gazette/broker:latest and
  gazette/examples:latest, instead of gazette-broker:latest and gazette-examples:latest.
• Additional sanity checks to ensure badly-behaved clients or peers cannot
  block an exiting broker from draining its server.
• Gazette no longer logs "wrong fragment format" warnings when encountering "directory" objects
  (those where the object key ends with a '/') in cloud storage. These were just adding noise and
  were not actually anything to worry about.
• Fix various test flakes.