- Move the thread-priority setting into the preprocess-job, so it's done earlier.
- Do less importing of contexts in cppparsejob, and flag temporary imports as such using a new flag.
- Add temporary imports using the new mechanism for that added to TopDUContext.

Together with the patches to TopDUContext, this greatly reduces the bottleneck in that area, so far that we can now start worrying about other stuff like memory-usage again.