Mesos master might drop some events in the operator API stream

Inside `Master::updateTask`, we call `Subscribers::send` which asynchronously calls `Subscribers::Subscriber::send` on each subscriber.

But the problem is that inside `Subscribers:Subscriber::send` we are looking up the state of the master (e.g., getting Task* and Framework*) which might have changed between `Subscribers::send ` and `Subscribers::Subscriber::send`.

For example, if a terminal task received an acknowledgement the task might be removed from master's state, causing us to drop the TASK_UPDATED event.

We noticed this in an internal cluster, where a TASK_KILLED update was sent to one subscriber but not the other.