Fix redaction authentication (#5700)

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.

1 files changed, 2 insertions, 0 deletions

diff --git a/changelog.d/5700.bugfix b/changelog.d/5700.bugfix
new file mode 100644
index 0000000000..51bce8d441
--- /dev/null
+++ b/changelog.d/5700.bugfix
@@ -0,0 +1,2 @@
+Fix some problems with authenticating redactions in recent room versions.
+