Backport(v1.16): server plugin helper: ensure to close all connections at shutdown (#5026)

[daipom]

Which issue(s) this PR fixes:

• Backport server plugin helper: ensure to close all connections at shutdown #5026

What this PR does / why we need it:
TCP server with server helper does not close all connections at shutdown process.
When receiving data from multiple clients, the server receive the data continuously because the connection is not closed.

The server will shut down properly by this PR

Reproduce

1. Launch Fluentd with following config file.
2. Send syslog data from two or more clients using following client script
3. Terminate Fluentd
4. Relaunch Fluentd, then it shows the 2025-07-16 14:12:26 +0900 [warn]: #0 restoring buffer file: path = xxxxxxxxx in logs.

• config

<source>
  @type syslog
  tag system

  <transport tcp>
  </transport>

  bind 0.0.0.0
  port 5140
</source>

<match **>
  @type file
  path "#{File.expand_path('~/tmp/fluentd/maillog')}"
  <buffer>
    @type file
    path "#{File.expand_path('~/tmp/fluentd/buffer/buffer_syslog_maillog')}"
    flush_at_shutdown true
  </buffer>
</match>

• client script

require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'remote_syslog_sender'
end

def create_client
  Thread.new do
    sender = RemoteSyslogSender.new('127.0.0.1', 5140, protocol: :tcp)
    loop do
      sender.transmit("message body")
      sleep 0.5
    end
  end
end

clients = []
3.times do
  clients << create_client
end

clients.each(&:join)

Example code.

@ary = [1,2,3]

# It would like 1, 2, and 3 to be processed.
# However, following code handles 1 and 3.
@ary.each do |i|
  puts i
  @ary.delete(i)
end

Docs Changes:
Not needed.

Release Note:
Same as the title.