Support the multi process worker configuration

[guisen4332]

When use the fluentd's multi-process workers feature like

<system>
  workers 3
</system>
<match **>
  type nsq
  buffer_type file
  buffer_path /var/log/fluent/msgbus
  nsqlookupd 127.0.0.1:4161
  topic logs
</match>

Errors like "Plugin 'nsq' does not support multi workers configuration (Fluent::Plugin::NSQOutput)" occured

This pr is for supporting the multi-process workers feature in the "NSQOutput"

[guisen4332]

@cosmo0920 Could you please review this PR? I don't have the necessary permissions to add reviewers directly