fix filters on a feature reference not matching any feature

[cportele]

A queryable for a feature reference is filtered against the id property of the reference. That only worked as long as none of the properties of the reference was mapped through a sub-decoder.

MappingRulesDeriver synthesizes a rule for the container of a value whose parent has no rule yet, and derived the target of that rule from the target of the value. For a sub-decoder column that is wrong: the column is not a property of the target schema, it is the container of the values that the following rules address, so its target has to be the root target. At the root that was the case by accident, because the target of a top-level value has no parent. Inside an object the container inherited the target of the object, the object was registered as a value, and it shadowed the id of the reference. The filter was then resolved against a member of the sub-decoder document named after the reference, which never exists.

• MappingRule.endsWithConnector tells a sub-decoder column from a join, MappingRulesDeriver uses it for the target of the container.
• SqlMappingDeriver registers only values as value targets, so an object can no longer shadow a value with the same target.
• The properties of a sub-decoder column are resolved from the schema the table is mapped from instead of from the root schema, so values in the sub-decoder document of a joined table are mapped, too. Their path in the document is now always recorded, it cannot be derived from the property name, e.g. the title of a feature reference may be mapped to any member.