Generates an entity name suitable for use with this document. If no logicalId is given (or if it doesn't map to an active session), a random number is used to create an entity name which is currently not in use by this document. Otherwise the associated CMS adapter session will be called to produce the entity name. The adapter guarantees that the returned entity name will be unique as per the given logicalId. Thus, if given the same logicalId twice, this may return the same entity name twice. However, if given different logicalId's, this will return different entity names.