Consider using entity type specialized entityreference selection classes

OgSelectionHandler always extends EntityReference_SelectionHandler_Generic, even though entity type specialized entityreference selection classes (such as EntityReference_SelectionHandler_Generic_node) exist.

For node entities, OgSelectionHandler overrides the only class overridden in EntityReference_SelectionHandler_Generic_node, so it is unlikely there is any functional change. For more exotic group types (user/term/file/comment) however, it seems likely that the fixes in the specialized classes (e.g. to improve label fetching) could improve behaviour.

For my specific case, the big win is in being able to use the patch in #2151631: Improve performance of getReferencableEntities(), which reduces memory usage from >400MB to 70MB on a user edit page for a user with many (1400) group memberships.