[K/N] Make STMS use separate threads for GC and finalizers.

Moves GC into a separate thread, and finalizers processing into yet
another thread. This makes STMS more similar to CMS.

kotlin-native/runtime/src/main/cpp/SingleLockList.hpp

@@ -48,7 +48,7 @@ public:
 
     class Iterator {
     public:
-        using difference_type = void;
+        using difference_type = std::ptrdiff_t;
         using value_type = Value;
         using pointer = Value*;
         using reference = Value&;