* MIGRATION
    - If stream_buffer or rate_limit is configured, please install
      "mbuffer" (version >= 20180505). Note that mbuffer is only
      required on remote hosts if stream_buffer_remote or
      rate_limit_remote is set.
    - In order to mimic old behavior, replace rate_limit with
      rate_limit_remote, and read btrbk.conf(5).
    - If you are using table output other than --format=raw in your
      scripts, note that the column headings changed from pretty
      two-line to (uppercase) one-line format. Either add --pretty
      option to mimic old behavior, or adapt your scripts.
  * Drop run-time dependency on "pv" (in favor of "mbuffer").
  * Combine stream_buffer and rate_limit: for rate_limit, use
    "mbuffer" (on local host) instead of "pv" (on source host).
  * Add stream_buffer_remote and rate_limit_remote options.
  * Use "mbuffer" instead of "pv" for --progress display.
  * Add stream_buffer functionality for raw targets.
  * Add action "ls": list all btrfs subvolumes below given path.
  * Change table output: print single-line, uppercase headings.
  * ssh_filter_btrbk.sh: whitelist mkdir if -t,--target option is set,
    used by "btrbk archive" action (close #297).
  * Bugfix: handle subvolumes with special characters (close #288).
  * Bugfix: don't display "<no_action>" in backup summary if volume is
    skipped by --exclude or noauto (close #291).
  * Bugfix: systemd: Use WantedBy=timers.target instead of
    WantedBy=multi-user.target in btrbk.timer (close #293).