In #17354 we made client updates prioritized to reduce client-to-server
traffic. When the client has no previously-acknowledged update we assume that
the update is of typical priority; although we don't know that for sure in
practice an allocation will never become healthy quickly enough that the first
update we send is the update saying the alloc is healthy.
But that doesn't account for allocations that quickly fail in an unrecoverable
way because of allocrunner hook failures, and it'd be nice to be able to send
those failure states to the server more quickly. This changeset does so and adds
some extra comments on reasoning behind priority.
78f4f76520