multiline_flush_interval does not work in combination with enable_watch_timer false

**Describe the bug**
In the case of a multiline tail log source, the last record in the file is not flushed after multiline_flush_interval has passed if the enable_watch_timer parameter is set to false. If enable_watch_timer parameter is set to true then all works as expected.

**To Reproduce**
Configure multiline tail source with multiline_flush_interval set to 10s and enable_watch_timer parameter set to false.
Prepare log file with several records.
Start fluentd and wait more than 10 seconds and then stop fluentd.
As a result first records flushed immediately but the last record flushed only during fluentd stopping.

**Expected behavior**
First records flushed immediately and last record flushed after 10 seconds according to multiline_flush_interval.

**Your Environment**

- fluentd 1.11.2
- Windows 10

**Your Configuration**
Configuration example:
```

@type tail
path C:\Temp\logs.txt
pos_file C:\Temp\posfile.pos

tag test
read_from_head true
multiline_flush_interval 10s
enable_watch_timer false

@type multiline
format_firstline /\[[^\]]*\] /
format1 /\[(?[^\]]*)\] \r?\n(?.*) \r?\n/

@type stdout

```

Log file example:
[logs.txt](https://github.com/fluent/fluentd/files/5600731/logs.txt)
```
[2020-10-19 12:21:33.8442]
1 Log message

[2020-10-19 12:22:33.8442]
2 Log message

```

**Your Error Log**

```
2020-11-26 12:24:02 +0700 [info]: adding match pattern="test" type="stdout"
2020-11-26 12:24:02 +0700 [info]: adding source type="tail"
2020-11-26 12:24:03 +0700 [info]: #0 starting fluentd worker pid=23040 ppid=11772 worker=0
2020-11-26 12:24:03 +0700 [info]: #0 following tail of C:\Temp\logs.txt
2020-10-19 12:21:33.844200000 +0700 test: {"message":"1 Log message"}
2020-11-26 12:24:03 +0700 [info]: #0 fluentd worker is now running worker=0
2020-11-26 12:24:49 +0700 [info]: Received graceful stop
2020-11-26 12:24:49 +0700 [info]: #0 fluentd worker is now stopping worker=0
2020-11-26 12:24:49 +0700 [info]: #0 shutting down fluentd worker worker=0
2020-11-26 12:24:49 +0700 [info]: #0 shutting down input plugin type=:tail plugin_id="object:ab4"
2020-10-19 12:22:33.844200000 +0700 test: {"message":"2 Log message"}
2020-11-26 12:24:49 +0700 [info]: #0 shutting down output plugin type=:stdout plugin_id="object:aa0"
2020-11-26 12:24:50 +0700 [info]: Worker 0 finished with status 0
```

**Additional context**
All works as expected if enable_watch_timer parameter is set to true.

Start by reproducing the multiline tail source configuration with multiline_flush_interval set to 10s and enable_watch_timer false, then compare it with the true setting. Trace the tail input and multiline parser behavior around timer-driven flushing; done means the final record is emitted after the interval without requiring Fluentd to stop, with coverage for both timer settings.

Written by the indexing model from the issue text.