Race conditions in stream unsubscription may lead to multiple
back-to-back SUBSCRIPTION_DEACTIVATED RealmAuditLog entries for the
same stream. The current logic constructs duplicate UserMessage
entries for such, which then later fail to insert.
Keep a set of message-ids that have been prep'd to be inserted, so
that we don't duplicate them if there is a duplicated
SUBSCRIPTION_DEACTIVATED row. This also renames the `message` local
variable, which otherwise overrode the `message` argument of a
different type.
6b6dcf6ce1