TVM multithread for windows

meet a problem that the program can not to break for ever when I use multithread with the vs2017 of windows.when i trace the code of runtime , finding the problem is located at ( %E5%BE%AE%E4%BF%A1%E6%88%AA%E5%9B%BE_20191122112804

this problem how to server ??

another problem is that when I not open the thread with vs2017, the destructor of workpool class can not be executed ,it can cause a leak memory.

@yidawang, @FrozenGene

@hcho3 If you have some spare circles, can you take a look at this issue on Windows? Let me know if you can reproduce the problem. Thanks.