formatter_json: use JSON as fallback parser instead of Yajl for performance

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
Recently, Ruby's json has incredible performance improvements.
It might be faster than oj gem.
So, I think json is a suitable as fallback.

This is similar with #4813

• Before

  • It spent 70.708872503 sec to handle 10 GB file

• After

  • It spent 54.428017716 sec to handle 10 GB file

• config

<source>
  @type tail
  path "#{File.expand_path('~/tmp/fluentd/access*.log')}"
  pos_file "#{File.expand_path('~/tmp/fluentd/access.log.pos')}"
  tag log
  read_from_head true
  <parse>
    @type none
  </parse>
</source>

<match **>
  @type file
  path "#{File.expand_path('~/tmp/fluentd/output/log')}"

  # use formatter_json
  format json
</match>

Docs Changes:

Release Note:

[daipom]

Thanks!