Introduced index improves select query performance. Example execution time of `selectSendToDeviceMessagesSQL` query dropped from 80 ms to 15 ms. No sytest modifications are required.
### Pull Request Checklist
* [x] I have added added tests for PR _or_ I have justified why this PR doesn't need tests.
* [x] Pull request includes a [sign off](https://github.com/matrix-org/dendrite/blob/main/docs/CONTRIBUTING.md#sign-off)
Signed-off-by: `Piotr Kozimor <p1996k@gmail.com>`