in_tail: reduce memory usage slightly

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
Previous, I changed lifespan of iobuf at #4763 to work GC collector well.

In addition, it seems that clearing iobuf manually when it is no longer needed, it might improve memory efficiency.

• config

<source>
  @type tail
  path "#{File.expand_path '~/tmp/access*.log'}"
  pos_file "#{File.expand_path '~/tmp/fluentd/access.log.pos'}"
  tag log
  refresh_interval 5s
  <parse>
    @type none
  </parse>
</source>

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

• script to generate log data

require "json"

path = File.expand_path("~/tmp/access.log")

File.open(path, "w") do |f|
  loop do
    log = { time: Time.now, message: "a" * 10_000 }.to_json
    f.puts log
    sleep 0.05
  end
end

Related to #4808

Docs Changes:

Release Note:

[daipom]

Thanks!

[daipom]

Mainly about #4763, I'm a little concerned about any unexpected negative effects on the processing speed of in_tail.
It would be good to check the changes in processing speed and memory consumption for the v1.19 release.