Fix inserted_at for seniority to Utc::now()

[macpie]

I do no think we need to fix current DB as this does not seem to impact much things.

• fetch_latest does not care about inserted_at.
• fetch_seniority is the only place where inserted_at is used to order seniority records. If we update the old fields we might alter the result or previous rewards run better leave the DB alone there IMO.
• I still need to check on proto records:

proto::SeniorityUpdate {
  key_type: Some(self.key.into()),
  new_seniority_timestamp: new_seniority.timestamp() as u64,
  reason: update_reason as i32,
  new_seniority_timestamp_ms: new_seniority.timestamp_millis() as u64,
},

The proto does not care about inserted_at anywhere I could find