First invitation over federation to a room fails with a 403, second works

This issue has been migrated from [#4345](https://github.com/matrix-org/synapse/issues/4345).

---

Both servers on Synapse 0.33.9.

@babolivier invited a test account on lant.uk to a direct chat on abolivier.bzh. The first invite failed - from the lant.uk logs:

```
2019-01-02 11:53:09,249 - synapse.http.matrixfederationclient - 384 - INFO - POST-3321894- {GET-O-13088713} [abolivier.bzh] Got response headers: 403 Forbidden
2019-01-02 11:53:09,250 - synapse.util.async_helpers - 218 - INFO - POST-3321894- Releasing linearizer lock 'member' for key (u'!zRkrJLyVJlGMRvPdkK:abolivier.bzh',)
2019-01-02 11:53:09,250 - synapse.http.server - 85 - INFO - POST-3321894- ' clientprot
o=u'HTTP/1.1' site=443> SynapseError: 403 - You are not invited to this room.
2019-01-02 11:53:09,251 - synapse.access.https.443 - 302 - INFO - POST-3321894- 80.250.101.234 - 443 - {@devbot:lant.uk} Processed request: 0.122sec/0.000sec (0.006sec, 0.000sec) (0.000sec/0.002sec/4) 85B 403 "POST /_matrix/cl
ient/r0/join/%21zRkrJLyVJlGMRvPdkK%3Aabolivier.bzh?access_token= HTTP/1.1" "python-requests/2.19.1" [0 dbevts]
```

The second invite (to a new dm in a new room) worked.

This actually happened twice - once as described on an invite from abolivier.bzh to lant.uk, once on an invite to a bot running on matrix.org from abolivier.bzh.