Review minimal effort approach on consumer last heartbeat on unsubscribe

Currently the legacy and new consumer follows a minimal effort approach when sending a leave group (legacy) or last heartbeat request (new consumer). The request is sent without waiting/handling any response. This behaviour applies when the consumer is being closed or when it unsubscribes.

For the case when the consumer is being closed, (which is a "terminal" state), it makes sense to just follow a minimal effort approach for "properly" leaving the group. But for the case of unsubscribe, it would maybe make sense to put a little more effort in making sure that the last heartbeat is sent and received by the broker. Note that unsubscribe could a temporary state, where the consumer might want to re-join the group at any time.