Solr search for property values for multivalued List of Users and List of Groups is not correctly identifying the multiple values

Steps to reproduce:

• create a class with a property of type list of users (or list of groups)
  • mark the property as "multiple select" (multivalue)
• create an object of the class in a page, fill in 2 users in the property
  • in the Sandbox.WebHome.xar there is such an example
• make a Solr search using advanced syntax for the pages with one of the users as value of that property, for example:

  property.Sandbox.TestClassWithUsers.users:XWiki.kbernick
  

Expected result

• the page with 2 users selected for the users list property is found as a result of this search

Actual result

• the page with 2 users selected for the users list property is not found as a result of this search
• however, if there is a page with an object that has a single value selected for the property, it will be found
• searching for

  property.Sandbox.TestClassWithUsers.users:"XWiki.kbernick,XWiki.jtonnesen"

  will find the page (when both values are included in the search, in order).

Note: this also impacts the possibility of setting up facets for these properties: they will not work properly if setup, they will filter for the full value (including the commas). So this is not only impacting the advanced search, it blocks faceting on these properties, which is more or less API of the solr search application.

This is because the multiple values of the users property are not indexed by Solr as 2 separate values, but as a single value, separated by comma. Ultimately this is because of the way list of users and list of groups properties are built, where relational storage cannot be activated regardless of what the user does (XWIKI-16197).
Also, the XWiki Solr metadata extractor doesn't have any kind of special handling of non-relational stored properties, even if, in the XWiki applications they behave as proper lists (as relational stored ones).