Harry multithreads SV_ComputeClientPacks to run 128 players at 64 tick

In #developer-talk, Harry said he multithreaded SV_ComputeClientPacks, increasing performance by over 50% and running a solid 64 tick with 128 players. He is looking into going past 128 players; he expects the entity limit to be the main problem, since it can't be raised without client changes.