fix(eventstore): prevent allocation of filtered events (#6749)

* fix(eventstore): prevent allocation of filtered events

Directly reduce each event obtained from a sql.Rows scan,
so that we do not have to allocate all events in a slice.

* reinstate the mutex as RWMutex

* scan data directly

* add todos

* fix(writemodels): add reduce of parent

* test: remove comment

* update comments

---------

Co-authored-by: adlerhurst <silvan.reusser@gmail.com>

internal/command/unique_constraints_model.go

@@ -183,7 +183,7 @@ func (rm *UniqueConstraintReadModel) Reduce() error {
 			rm.removeUniqueConstraint(e.Aggregate().ID, e.UserID, member.UniqueMember)
 		}
 	}
-	return nil
+	return rm.WriteModel.Reduce()
 }
 
 func (rm *UniqueConstraintReadModel) Query() *eventstore.SearchQueryBuilder {