Provide chunk.key for v0.12 plugins which specifies key in overriding #emit

[tagomoris]

3rd party BufferedOutput plugins sometimes overrides #emit to control chunking keys.
This change is to provide compatibility for such plugins by using metadata.variables to pass specified chunking key to #write.

This fix solves #990.

[tagomoris]

I checked this change with fluent-plugin-sql and configuration below:

<source>
  type dummy
  tag test
</source>

<match test>
  @type sql
  host localhost
  port 5432
  database fluentd_test
  adapter postgresql
  username fluentd
  password fluentd
  <table>
    table mytest
    column_mapping message:message
  </table>
  <table test>
    table mytest
    column_mapping message:message
  </table>
</match>