Entries in CatalogUsageMonitor are not cleared after invalidation

The CatalogUsageMonitor in the catalog maintains a small cache of references to tables that: a) are accessed frequently in the catalog and b) have the highest memory requirements. These entries are not cleared upon server or table invalidation, thus preventing the GC from collecting the memory of these tables. We should make sure that the CatalogUsageMonitor does not maintain entries of tables that have been invalidated or deleted.