graceful shutdown looks like lose the received events

- fluentd or td-agent version.
fluentd 1.3.3

- Environment information
linux
ruby 2.5.1

- configuration
```

@type forward


@type file
path "/tmp/fluent-shutdown-test/output/${tag}"
append true

@type "memory"
path /tmp/fluent-shutdown-test/output/${tag}


```

* test code - https://github.com/HatsuneMiku3939/fluent-shutdown-test/blob/master/test.rb
* https://travis-ci.org/HatsuneMiku3939/fluent-shutdown-test/builds/477185717

graceful shutdown immediately after post a large number of events looks like lose received events.

- case 1
post 10 events -> graceful shutdown immediately -> file output contains 10 events

- case 2
post 100000 events -> graceful shutdown immediately -> file output doesn't contains 100000 events

- desired behavior of case 2
post 100000 events -> graceful shutdown immediately -> file output contains 100000 events