Unhandled Error: doWrite called on a twisted.internet.udp.Port

This issue has been migrated from [#4727](https://github.com/matrix-org/synapse/issues/4727).

---

I've saw this message in my error log:

```
twisted: [] Unhandled Error
Traceback (most recent call last):
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/twisted/python/log.py", line 103, in callWithLogger
return callWithContext({"system": lp}, func, *args, **kw)
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/twisted/python/log.py", line 86, in callWithContext
return context.call({ILogContext: newCtx}, func, *args, **kw)
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/twisted/python/context.py", line 122, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/twisted/python/context.py", line 85, in callWithContext
return func(*args,**kw)
--- ---
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/twisted/internet/posixbase.py", line 619, in _doReadOrWrite
why = selectable.doWrite()
File "/opt/venvs/matrix-synapse/lib/python3.5/site-packages/twisted/internet/base.py", line 1192, in doWrite
"doWrite called on a %s" % reflect.qual(self.__class__))
builtins.RuntimeError: doWrite called on a twisted.internet.udp.Port
```

### Version information

- **Version**: matrix-synapse-py3 0.99.1.1+stretch1
- **Install method**: Debian package from matrix.org
- **Platform**: amd64, Debian stable