Dendrite doesn't /federation/send properly to Construct.

*This issue was originally created by [**@su-ex**](https://github.com/su-ex) at .*

### Background information

- **Dendrite version or git SHA**: any
- **Monolith or Polylith?**: any
- **SQLite3 or Postgres?**: any
- **Running in Docker?**: any
- **`go version`**: any

### Description

### Steps to reproduce

Have a room with a Dendrite and a construct server.

It seems Dendrite just goes into backoff:
```
level=debug msg="Sending transaction \"1626775843222-5\" containing 0 PDUs, 1 EDUs" func="nextTransaction\n\t" file=" [destinationqueue.go:409]" server_name=some.dendrite.server
level=warning msg="Backing off \"construct.supercable.onl\" for 15.868026157s" func="backgroundSend\n\t" file=" [destinationqueue.go:301]"
```

This means construct will only get events from Dendrite through backfill via other servers.

Reproduce with a room spanning Dendrite and a Construct server, then inspect the federation send path around destinationqueue.go:301 and destinationqueue.go:409, including backgroundSend and nextTransaction. Use the reported backoff logs to identify why /federation/send fails; done means Construct receives events directly rather than only through backfill.

Written by the indexing model from the issue text.