Ensures that redactions are correctly authenticated for recent room versions.
There are a few things going on here:
* `_fetch_event_rows` is updated to return a dict rather than a list of rows.
* Rather than returning multiple copies of an event which was redacted
multiple times, it returns the redactions as a list within the dict.
* It also returns the actual rejection reason, rather than merely the fact
that it was rejected, so that we don't have to query the table again in
`_get_event_from_row`.
* The redaction handling is factored out of `_get_event_from_row`, and now
checks if any of the redactions are valid.
