Backport(v1.16): formatter_csv: fix memory leak (#4864)

[kenhys]

Which issue(s) this PR fixes:
Backport #4864
Fixes #3627

What this PR does / why we need it:
Seems in_exec plugin creats Thread object each interval and it invoke formatter_csv on the thread.

If it use a Thread object as hash key when caching with formatter_csv, that thread will be permanently referenceable and will not be collected by the GC.

Here is memory usage of Ruby's process.

Reproduce

See #3627

<system>
  process_name fluentd_monitor
</system>

<source>
  @type exec
  tag hardware.memory
  run_interval 0.1
  command /home/watson/prj/sandbox/fluentd/get_memory_metrics.sh
  <parse>
    @type json
    types mem_available:float,swap_total:float,swap_free:float
  </parse>
</source>

<match **>
  @type exec
  command /home/watson/prj/sandbox/fluentd/psql_memory.sh
  <format>
    @type csv
    force_quotes false
    delimiter |
    fields time_local,mem_available,swap_total,swap_free
  </format>
  <buffer time>
    @type file
    path /home/watson/prj/sandbox/fluentd/log
    timekey 50s
    timekey_wait 0s
    flush_mode lazy
    total_limit_size 200MB
    retry_type periodic  # exponential backoff is broken https://github.com/fluent/fluentd/issues/3609
    retry_max_times 0  # need to go to secondary as soon as possible https://github.com/fluent/fluentd/issues/3610
  </buffer>
</match>

Docs Changes:

Release Note:
formatter_csv: fix memory leak