MXS-1360 Make it possible to specify thread stack size

It is now possible to specify the thread stack size to be used,
when a new thread is created. This will subsequently be used
for allowing the stack size to be specified for worker threads.

server/core/test/testatomic.cc

@@ -82,7 +82,7 @@ int run_test(void(*func)(void*))
 
     for (size_t i = 0; i < NTHR; i++)
     {
-        if (thread_start(&threads[i], func, (void*)(i + 1)) == NULL)
+        if (thread_start(&threads[i], func, (void*)(i + 1), 0) == NULL)
         {
             ss_dassert(false);
         }