some task queueing improvements:

- do not pass CHILD_SAs to task constructor, might not
	  be valid anymore during execution (late lookup)
	- use sub-tasks to delete CHILD/IKE_SA after rekeying,
	  as we want to execute the delete before additional
	  queued tasks

src/charon/sa/tasks/child_create.c

@@ -1119,6 +1119,8 @@ static void migrate(private_child_create_t *this, ike_sa_t *ike_sa)
 	}
 	
 	this->ike_sa = ike_sa;
+	this->keymat = ike_sa->get_keymat(ike_sa);
+	this->proposal = NULL;
 	this->proposals = NULL;
 	this->tsi = NULL;
 	this->tsr = NULL;