This is the first part of the (dynamic) thread boost implementation, which boosts the main thread only.
In my testing on Windows just being the main thread usually gives a +1 to +2 priority boost (which also plays well with the theoretical QOS boost that main threads on macOS also have).
I assume on Linux this will be similarly beneficial in some cases.
This is a conservative +1 boost to the main thread for now. The next (and final part) will boost the priority of the thread handling the foreground windows messages (which is also something that native Windows does).
It is hard to reliably write tests for these adjustments since there is some more dynamic behavior, but in my testing this approach is a pretty good approximation (and does improve performance on some games like GTA V measurably).
-- v2: server: Boost main thread priority. server: Add set_thread_disable_boost() helper.