Fix memory usage

[Watson1978]

This should fix fluent/fluentd#3401

To reduce memory usage,
this uses a feature added by ledbettj/systemd-journal#103

Result

conf

<source>
  @type systemd
  tag systemd
  path /var/log/journal
  read_from_head true

  <entry>
    fields_strip_underscores true
    fields_lowercase true
  </entry>

  @label @OUTPUT
</source>

<label @OUTPUT>
  <match **>
    @type file
    path /var/log/fluentd-output
    compress gzip
    <buffer>
      timekey 1d
      timekey_use_utc true
      timekey_wait 10m
    </buffer>
  </match>
</label>

systemd journal generator

msg = "a" * 120 * 1024

loop do
  system("logger '#{msg}'")
end

[kenhys]

LGTM.