prefix.relationKeys set is not handled correctly on unlink which then breaks unlinkAll

[maritz]

The set of prefix.relationKeys (where keys are stored under which the actual relations then are) is currently not handled properly on an unlink. It removes the entire thing.

Instead it should probably remove the ids from the prefix.relations set as it is now, then in an asynchronous check afterwards conditionally remove the prefix.relationKeys set, if the prefix.relations set is empty/gone.