Collect active query jobs into struct `QueryJobMap`
This PR encapsulates the existing `QueryMap` type alias into a proper struct named `QueryJobMap`, which is used by code that wants to inspect the full set of currently-active query jobs.
Wrapping the query job map in a struct makes it easier to see how other code interacts with the map, and also lets us change some free functions for map lookup into methods.
We also do a renaming pass to consistently refer to the query job map as `job_map`, or as `job_map_out` in the places where it's a mutable out-parameter.
There should be no change to compiler behaviour.
r? nnethercote (or compiler)
Stuart Cook
504bc6699c