NoneType has no attribute shutdown

This issue has been migrated from [#3837](https://github.com/matrix-org/synapse/issues/3837).

---

Unsure what causes this, but it seems to happen frequently. Observed on a `federation_sender` worker. Shows up in pairs.

```
federation_sender_1 - 2018-09-10 20:03:46,477 - twisted - 131 - CRITICAL - - Unhandled Error
Traceback (most recent call last):
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/synapse/app/_base.py", line 104, in run
reactor.run()
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/internet/base.py", line 1261, in run
self.mainLoop()
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/internet/base.py", line 1270, in mainLoop
self.runUntilCurrent()
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/synapse/metrics/__init__.py", line 243, in f
ret = func(*args, **kwargs)
--- ---
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/internet/base.py", line 896, in runUntilCurrent
call.func(*call.args, **call.kw)
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/synapse/util/async_helpers.py", line 415, in time_it_out
deferred.cancel()
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 548, in cancel
self.result.cancel()
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 537, in cancel
canceller(self)
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/web/_newclient.py", line 1481, in cancelRequest
self.transport.abortConnection()
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/protocols/tls.py", line 435, in abortConnection
self._shutdownTLS()
File "/home/matrix/.synapse/local/lib/python2.7/site-packages/twisted/protocols/tls.py", line 338, in _shutdownTLS
shutdownSuccess = self._tlsConnection.shutdown()
exceptions.AttributeError: 'NoneType' object has no attribute 'shutdown'
```