No cache invalidation after object relation update

_Scenario_: An object of type analogsignal is attached to a specific segment by updating the analogsignal's "segment" field. Retrieving the affected segment results in a new entry under "analogsignal_set" (as intended). However, the ETag of the segment remains unaltered, presumably due to "passive" relationship information only being established upon request. This results in undesirable behavior where the segment still appears to be attached to the analogsignal whereas requesting the analogsignal shows otherwise.

_Proposed solution_: Passive changes (i.e., relationship changes as a consequence of changes to other objects) should invalidate the ETag of the affected object.