Queue fix

Separate adding and getting a task. Uses a double-ended queue to buffer incoming tasks. The buffer is then added to the actual queue whenever queue_gettask is called.

Peter, can you check if this doesn't break anything? Thanks!

src/atomic.h

@@ -26,5 +26,6 @@
 #define atomic_inc(v) atomic_add(v, 1)
 #define atomic_dec(v) atomic_add(v, -1)
 #define atomic_cas(v, o, n) __sync_val_compare_and_swap(v, o, n)
+#define atomic_swap(v, n) __sync_lock_test_and_set(v, n)
 
 #endif /* SWIFT_ATOMIC_H */