Another fix for #358: async thr wait in basic_client_facade dtor

Sometimes, it has crashed in my tc after #402.

*** glibc detected *** ./TestHttp: double free or corruption (out): 0x00007fe760001f70 ***

It's maybe a race condition.
So, I've rewritten by async thr wait in basic_client_facade dtor.
It perfectly works as following http_temp_client_get_test.
Why does this need to be virtual?