Fix destructor of async loop, join thread, terminate properly.
Fix destructor of async loop, join thread, terminate properly.

MengjiaoH/ospray,ospray/OSPRay,MengjiaoH/ospray,MengjiaoH/ospray,ospray/OSPRay,ospray/OSPRay,ospray/OSPRay,wilsonCernWq/ospray,MengjiaoH/ospray,MengjiaoH/ospray,wilsonCernWq/ospray,wilsonCernWq/ospray