optimize query to determine whether mc db exists
optimize query to determine whether mc db exists

the existing query using information_schema.tables was taking about
200ms for every new connection, generating about half of the load on the
postgres server.  this new version takes less than 1ms.

berkmancenter/mediacloud,berkmancenter/mediacloud,berkmancenter/mediacloud,berkmancenter/mediacloud,berkmancenter/mediacloud