db: Ignore failed constraint when saving events
This fixes the following warning that occurs when multiple accounts share rooms and therefore events:
cm-db[27063]: WARNING: Failed to save event: $17235795164205fqkrG:matrix.org, error: UNIQUE constraint failed: room_events.room_id, room_events.event_uid
This is an alternative to !97 that does not require any schema changes and related DB migrations.