Federation: /rooms/.../members fails for the invites received from a federated server

This issue has been migrated from [#4985](https://github.com/matrix-org/synapse/issues/4985).

---

**Description**
get `/_matrix/client/r0/rooms/{roomId}/members` on an invite received from another federated server fails with the following error:
```
errcode = "M_NOT_FOUND";
error = "Can't find event for token Token(room_key=u'xxx', presence_key=u'xxx', typing_key=u'x', receipt_key=u'xxx', account_data_key=u'xxx', push_rules_key=u'xx', to_device_key=u'xxx', device_list_key=u'xxx', groups_key=u'xx')";
```
**Steps to reproduce**
Consider 2 users userA and userB, created on 2 different Homeservers.
userA creates a room and invites userB
When userB requests for the members of this room, he gets an error