Update/Reload without downtime

[daipom]

Which issue(s) this PR fixes:

• Fixes #4622

What this PR does / why we need it: See #4622.

Specification:

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.

Supported input plugins:

• in_tcp
• in_udp
• in_syslog

Needs following:

• 4661

  • (Included in this branch temporarily)
• https://github.com/treasure-data/serverengine/pull/150

Docs Changes: TODO

Release Note: TODO

TODO:

• [ ] Some implementation TODO referred in code comment.
• [ ] Tests
• [ ] Document

[daipom]

The basic implementation is done. Some concept of #4654 is reflected. Thanks @Watson1978!