Major Description
A new event for heartbeat is added in MESOS-7695, but I believe the implementation in https://reviews.apache.org/r/61262/ can trigger a corner case and send HEARTBEAT before SUBSCRIBED
I would consider this a behavior change for the customer and I propose we change the order as I suggest in the review to preserve previous behavior (since the subscriber needs to see the SUBSCRIBED event to really know how it should respond to HEARTBEAT message anyway)