fluentd command: add --with-source-only

[daipom]

Which issue(s) this PR fixes:
None.
Related to

• Update/Reload without downtime #4622

This feature may be useful on its own, but mainly, this feature will help #4622 feature.

What this PR does / why we need it:
Add --with-source-only option to fluentd command.
(Ref: Similar to existing --without-source option)

It launches Fluentd with Input plugins only.
Here is the specification.

• Those Input plugins emits the events to SourceOnlyBufferAgent.
• The events is kept in the buf_file during the source-only-mode.
• SIGRTMIN(34) cancels the mode.
• After canceled, the new agent starts to load the buffer.

Docs Changes:
TODO

Release Note:

• fluentd command: add --with-source-only
• embedded plugin: add out_buffer plugin, which can be used for buffering and relabeling events.

new plugin: out_buffer

This feature internally uses the new plugin out_buffer.
This plugin can be used for buffering and relabeling events.

Example: in_udp -> out_buffer -> out_stdout

<source>
  @type udp
  tag foo.udp
  @label @buffer
  <parse>
    @type none
  </parse>
</source>

<match foo.**>
  @type stdout
</match>

<label @buffer>
  <match **>
    @type buffer
    @label @ROOT
    <buffer>
      path /path/to/buffer
    </buffer>
  </match>
</label>

TODO

• Consider race condition
• Fix and add tests
• Consider how to stop the loading agent
• Consider configurablity of SourceOnlyBufferAgent.
• Consider Windows support