[MONDRIAN-1998] Fixes a problem with the lookups in the RolapStarRegistry when external events are processed. When we build segments, we use the alias of the relation. The same should be done when we process external cache events and tie them to the rolap schemas they belong to. Not doing so leads to unpredictable results, since the implementation of toString() might prepend the schema name before the table name and cause a mismatch and cause the external segments not to be indexed in their respective rolap star indexes.