This replaces the current `SIGUSR2` (#2716) with the new feature.
(Not supported on Windows).

* Restart the new process with zero downtime

The primary motivation is to enable the update of Fluentd
without data loss of plugins such as `in_udp`.

Specification:

* 2 ways to trigger this feature (non-Windows):
  * Signal: `SIGUSR2` to the supervisor.
    * Sending `SIGUSR2` to the workers triggers the traditional
      GracefulReload.
      * (Leave the traditional way, just in case)
  * RPC: `/api/processes.zeroDowntimeRestart`
    * Leave `/api/config.gracefulReload` for the traditional feature.
* This starts the new supervisor and workers with zero downtime
  for some plugins.
  * Input plugins with `zero_downtime_restart` supported work in
    parallel.
    * Supported input plugins:
      * `in_tcp`
      * `in_udp`
      * `in_syslog`
  * The old processes stop after 10s.
* The new supervisor works in `source-only` mode (#4661)
  until the old processes stop.
  * After the old processes stop, the data handled by the new
    processes are loaded and processed.
  * If need, you can configure `source_only_buffer` (see #4661).
* Windows: Not affected at all. Remains the traditional
  GracefulReload.

Mechanism:

1. The supervisor receives SIGUSR2.
2. Spawn a new supervisor.
3. Take over shared sockets.
4. Launch new workers, and stop old processes in parallel.
   * Launch new workers with source-only mode
     * Limit to zero_downtime_restart_ready? input plugin
   * Send SIGTERM to the old supervisor after 10s delay from 3.
5. The old supervisor stops and sends SIGWINCH to the new one.
6. The new workers run fully.

Note: need these feature

* #4661
* treasure-data/serverengine#146

Conditions under which `zero_downtime_restart_ready?` can be enabled:

* Must be able to work in parallel with another Fluentd instance.
* Notes:
  * The sockets provided by server helper are shared with the
    new Fluentd instance.
  * Input plugins managing a position such as `in_tail` should
    not enable its `zero_downtime_restart_ready?`.
    * Such input plugins do not cause data loss on restart, so
      there is no need to enable this in the first place.
  * `in_http` and `in_forward` could also be supported.
    Not supporting them this time is simply a matter of time to
    consider.

The appropriateness of replacing the traditional SIGUSR2:

* The traditional SIGUSR2 feature has some limitations and issues.
  * Limitations:
    1. A change to system_config is ignored because it needs to
       restart(kill/spawn) process.
    2. All plugins must not use class variable when restarting.
  * Issues:
    * #2259
    * #3469
    * #3549
* This new feature allows restarts without downtime and such
  limitations.
  * Although supported plugins are limited, that is not a
    problem for many plugins.
    (The problem is with server-based input plugins where the
    stop results in data loss).
* This new feature has a big advantage that it can also be used
  to update Fluentd.
  * In the future, fluent-package will use this feature to allow
    update with zero downtime by default.
* If needed, we can still use the traditional feature by RPC or
  directly sending `SIGUSR2` to the workers.

Co-authored-by: Shizuo Fujita <[email protected]>
Co-authored-by: Kentaro Hayashi <[email protected]>
Signed-off-by: Daijiro Fukuda <[email protected]>