Use `GROUP_CONCAT` to minimize result row count blowup

[ledsoft]

When optimizing entity query result loading, specifying additional attributes may lead to a blowup of the number of result rows because rows represent cartesian product of the different column values (for matching subjects). SPARQL 1.1 GROUP_CONCAT may be in certain cases used to reduce this effect by concatenating values into one row. Apply this optimization in relevant cases such as:

• plural attribute whose value are not references to other entities (already implemented in Enhancement#425 entity graph fetch #433)
• multilingual string attribute values (Enhancement#434 group concat multilingual strings #440)
• plural string attributes ([Enhancement #434] Support plural string attribute group_concat. #441)