supercharge dynFrame

we now have the capacity to suggest the relative importance of a given thread. A thread can be:
- urgent, meaning it will run every frame for 15 seconds before relinquishing control
- normal, meaning it will run every frame for 20 milliseconds
- background, meaning it will run every frame for at most 2 milliseconds

Login thread is the only thread expected to be urgent, as the experience of spillover from loading screen into contentful frames kind of sucks. background priority is currently unused, though migrating snapshots & spell cache to background priority threads seems appealing

additionally, add capability for threads to indicate how much time the next action will probably take
thread runner will use that time estimate to determine if it should defer the next action until the next frame
default is 1000 ms for urgent threads, 1 ms for normal threads, and 0.5 ms for background threads