Rework the ThreadPool, creating one task queue per player

[IntegratedQuantum]

remaining work:

• double check the code
• figure out a better metric to prioritize between taking normal work and taking work from the player queue, apparently a 50/50 split is not working very well. Maybe we should just always prioritize the other queue?
• benchmark overhead of sorting tasks on master vs this while moving over terrain with hyperspeed
• check if the thread idle time has increased significantly
• verify that it limits data as it's supposed to do under poor network conditions

fixes #418
fixes #399