ReservedTicker table and query is unsound

[erikd]

The current table only contains the ticker name and the pool_hash and the current query is:

queryReservedTicker :: MonadIO m => Text -> ReaderT SqlBackend m (Maybe ReservedTicker)
queryReservedTicker reservedTickerName = do
  res <- select . from $ \(reservedTicker :: SqlExpr (Entity ReservedTicker)) -> do
            where_ (reservedTicker ^. ReservedTickerName ==. val reservedTickerName)
            limit 1
            pure $ reservedTicker
  pure $ fmap entityVal (listToMaybe res)

The database does not store pool deregistations (so it is not possible to check for a deregistered pool) and the query does not take into account the possibility that the pool has been deregistered.