[GSS] Elytron insufficiently covers scenarios with more user storages - Multiple Realm Aggregation

We report this issue as feature regression between Elytron and PicketBox in following scenario:
Identity can be stored in more different identity storage (storage1 and storage2). Application server should provide configuration which is able to search user in both storage1 and storage2. It should be independent on underlying data storage - e.g. users can be stored in LDAP and Database, in two or more different LDAP servers etc.

As GSS confirm in [1], usage of two different LDAP servers for authentication is very common scenario used by customers.

This feature is currently provided by legacy security (PicketBox), but is missing feature in Elytron. It seems that Elytron is missing any sufficient form of Multiple Realm Aggregation.

We request blocker since this is common scenario used by customers. Missing this feature in Elytron can cause that customers will not be able to migrate to Elytron without changing their data.

[1] https://issues.jboss.org/browse/JBEAP-8734?focusedCommentId=13362843&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13362843