Previous method only worked for a normal shutdown, and didn't include
when the server enters a closing state due to watchdog crashes
This is the correct variable to detect the server is in the middle of shutdown process
In most cases, this change won't benefit much. However, there
exists the possibility that your Chunk Task threads are all busy
doing super slow work such as converting chunks.
If this occurs, the main thread blocking tasks, even at highest priority,
has to wait for some thread to become available.
This change gives us a waiting thread used only for main thread blocking
tasks, as well as an increased thread priority level, so that the OS
will give priority to this thread over the other threads.
This is more about guarantees, and won't be any real performanc boost
to anyone who has low or fast activity on their chunk tasks anyways.
But not all of us force upgrade our worlds, and this can be a life saver.
also reordered some patches because multiple PR's were merged.
