Follow the spec regarding /client/r0/keys/changes

This issue has been migrated from [#3058](https://github.com/matrix-org/synapse/issues/3058).

---

### Description

Moving the discussion from the spec repo (https://github.com/matrix-org/matrix-doc/issues/1170).

A request to the endpoint will succeed even if it's missing the `to` query parameter which is marked as mandatory.

### Steps to reproduce

```bash
curl -XGET "https://matrix.org/_matrix/client/r0/keys/changes?access_token=${TOKEN}&from=${NEXT_BATCH}"
```
Replacing `TOKEN` and `NEXT_BATCH` with a valid access token and a next_batch token returned by sync, respectively.

### Expected behavior.

The endpoint should throw an error.

### Version information

Tested with synapse v0.26 & with matrix.org