DNS failures connecting to IP address (!) for replication

This issue has been migrated from [#9865](https://github.com/matrix-org/synapse/issues/9865).

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We had a slew of this sort of error:

```
2021-04-22 10:12:31,676 - synapse.http.client - 453 - INFO - PUT-1883814-- - Error sending request to POST http://10.103.0.1:8135/_synapse/replication/fed_send_edu/m.direct_to_device/cYKChokZoY: DNSLookupError Couldn't find the hostname '10.103.0.1'
```

There are two things of concern here:
* we are doing DNS lookups for IP addresses. This is somewhat known, though highly unsatisfactory. It happens within `HostnameEndpoint` in twisted, which is used fairly widely, including the HTTP Agent. (Raised as https://twistedmatrix.com/trac/ticket/10181)
* the fact the DNS lookup failed.