Use of deprecated API

[achntrl]

Describe what happened:

We use BufferedOutput:

https://github.com/fluent/fluentd/blob/master/lib/fluent/compat/output.rb#L205

That will be deprecated at some point (don't know when)

We should proactively migrate to the new API and make sure nothing breaks before it bites us and we're forced to migrate

cf https://www.fluentd.org/blog/fluentd-v0.14.12-has-been-released in the "Making plugins available with multi process workers" section

[gaetan-deputier]

Resolved with 0.12.0