v0.89.0

• Optional consumer.MessageProducer interface allows applications
  to provide an alternate driver of message reading & parsing,
  outside of the built-in mechanisms for reading journals named
  in the ShardSpec.
• Binary-compatible enhancement of recovery logs allows
  for cheap "forks", where historical content of one log is used
  from within another.
  This introduced a breaking change to the recoverylog.Recorder,
  which now has a dedicated constructor and non-exported fields.
• Allow journal names in Read and Append RPCs to be extended with a
  'name;suffix', where the suffix is captured into consumer checkpoint offsets.
  This allows a single consumer to have multiple, concurrent reads of a journal.
• New instrumentation for testing and building custom consumer executables.
• Fix a memory leak in append flow control.
• Fix an allocator bug which could cause delayed convergence under pretty uncommon conditions.
• Fix some corner cases on timely reporting of shard progress in the Stat RPC.
• Update the versions of all dependencies.