Parallel reads for expensive master v1 read-only calls.

Similar to MESOS-9158 - we should make the operator API calls which serve master state perform computation of multiple such responses in parallel to reduce the performance impact on the master actor.

Note that this includes the initial expensive SUBSCRIBE payload for the event streaming API, which is less straightforward to incorporate into the parallel serving logic since it performs writes (to track the subscriber) and produces an infinite response, unlike the other state related calls.