GS4 query / net.minecraft.server.MinecraftServer.getPlayerNames() is not thread-safe

The bug

net.minecraft.server.MinecraftServer.getPlayerNames() (Mojang name) is not thread-safe. This can be problematic because it implements net.minecraft.server.ServerInterface.getPlayerNames() for the DedicatedServer. That method is called from a different thread when using GS4 query, see QueryThreadGs4.