* Populate `internal_metadata.outlier` based on `events` table
Rather than relying on `outlier` being in the `internal_metadata` column,
populate it based on the `events.outlier` column.
* Move `outlier` out of InternalMetadata._dict
Ultimately, this will allow us to stop writing it to the database. For now, we
have to grandfather it back in so as to maintain compatibility with older
versions of Synapse.
