DS-2248 XMLUI Delete Community doesn't work

https://jira.duraspace.org/browse/DS-2248

This fixes two things:  (1) have DSO subclass constructors ensure that the embedded TableRow has a table name, to prevent NPE later; (2) decache Collection much later in delete() to prevent the object reappearing in the Context cache and causing spurious test failures.  Plus some tidying-up of surrounding code.