To make it easier to re-use plugin shutdown process.

Signed-off-by: Daijiro Fukuda <[email protected]>

Add a new feature: Update/Reload without downtime.

1. The current supervisor receives a signal.
2. The current supervisor sends signals to its workers, and the
   workers stop all plugins that cannot run in parallel.
3. The current supervisor starts a new supervisor.
   * => Old processes and new processes run in parallel.
4. After the new supervisor and its workers start to work, the
   current supervisor and its workers stop.

ref: nginx's feature for upgrading on the fly

* http://nginx.org/en/docs/control.html#upgrade

Problem to solve:

Updating Fluentd or reloading a config causes downtime.
Plugins that receive data as a server, such as `in_udp`, `in_tcp`,
and `in_syslog`, cannot receive data during this time.
This means that the data sent by a client is lost during this
time unless the client has a re-sending feature.
This makes updating Fluentd or reloading a config difficult in
some cases.

Specific feature:

Run only limited Input plugins in parallel, such as `in_tcp`,
`in_udp`, and `in_syslog`.
Stop all plugins except those Input plugins, and prepare a
dedicated file buffer for Output.
After the new workers start, they load the file buffer and route
those events to the ROOT label.

Note: need treasure-data/serverengine#146

Signed-off-by: Daijiro Fukuda <[email protected]>

TODO: This patch should be removed. We need to make a specific
plugin for this feature.

Signed-off-by: Daijiro Fukuda <[email protected]>