added RelationDespawnMarker, rollback UnsetAsymmetric command changes

[aggyomfg]

Hello! Looks like I messed up in #25 :)

My main problem that I tried to solve initially was to use relations that can point each other (and first that I faced - was event duplication, but this wasn't root cause), later I get in situation when i unset relations from entity_a, it also unsets from entity_b (if they points each-other). And I found that we remove Hosts/Targets in UnsetAsymmetric command when they are empty, and this triggers on_remove hooks. But this doesnt fit for Orphan/Counted, so I added RelationDespawnMarker for this types with same hook.

still work in progress

[aggyomfg]

Now I think it's ready for review, I rolled back UnsetAsymmetric #25 changes (but removed target_exists checks, cause it make no sense and broke some event type)

And improved tests to check all this scenarios.