synapse.http.site - WARNING - Error processing request …: <class 'twisted.internet.error.ConnectionDone'> Connection was closed cleanly.

This issue has been migrated from [#5679](https://github.com/matrix-org/synapse/issues/5679).

---

### Description

I'm seeing many warnings like these in the log, which isn't a bad case, i.e. nothing to be warned about:

```
2019-07-13 10:59:43,933 - synapse.http.site - 203 - WARNING - GET-295130- Error processing request : Connection was closed cleanly.
2019-07-13 10:59:43,934 - synapse.access.http.8083 - 302 - INFO - GET-295130- 2a02:810a:87c0:3a11:aefd:ceff:fe3d:9e83 - 8083 - {@joerg:alea.gnuu.de} Processed request: 0.004sec/0.131sec (0.004sec, 0.004sec) (0.000sec/0.000sec/0) 364B 200! "GET /_matrix/client/r0/sync?filter=8&timeout=0&since=s2661503_51275966_31471_1968800_52703_4_10151_380580_8 HTTP/1.0" "Mozilla/5.0 (X11; Linux x86_64; rv:70.0) Gecko/20100101 Firefox/70.0" [0 dbevts]
```

Patch: [synapse-con-done.txt](https://github.com/matrix-org/synapse/files/3388956/synapse-con-done.txt)

### Version information

- **Version**: 1.0.0-2
- **Install method**: `apt install matrix-synapse`
- **Platform**: Debian/stable