Keep receiving messages even after recieveing SIGTERM and lost data

Even after fluentd receives SIGTERM and closes, fluentd seems to keep recieving messages for a while.
So, even if `flush_at_shutdown` is `true`,
* buffer remains after shutdown
* some messages seem to be lost

------------

Environment:

commit `9738a421c2fe9f001b723ce86e4a6e47412c4baf`
ruby 2.3.1p112
Mac OS 10.13.4

Preparation:
```
gem install fluent-logger

vi fluentd.conf
vi post.rb # source is as follows
```

fluentd.conf is
```fluentd.conf

@type forward

@type file
path tmp/file
flush_at_shutdown true

```

post.rb is
```post.rb
require 'fluent-logger'
log = Fluent::Logger::FluentLogger.new

cnt = 0
while log.post('debug', { message: cnt+1 })
cnt += 1
sleep 0.00001
end
puts "finished at #{Time.now}, post-count: #{cnt}"
```

Run commands below:
```
bundle exec ./bin/fluentd -c ./fluent.conf &
# => [1] 61688
ruby post.rb &
kill 61688
# => ....
# => finished at 2018-05-16 13:39:10 +0900, post-count: 111296
```

After that, you can find a buffer file.
```
tail -n 1 tmp/file.20180516_0.log
# => 2018-05-16T13:39:07+09:00 debug {"message":79760}
tail -n 1 tmp/file/buffer.b56c4b4cd48555a7da3dd7abe659bf157.log
# => 2018-05-16T13:39:07+09:00 debug {"message":79763}
```
In this case, messages between 79763 and 111296 seem to be lost.