GC fixes/improvements

- Have a real finalizer stage for destroying objects instead of mixing them
  in to the sweep stage.
- Base GC rate on a running average of the allocation rate instead of basing
  it on the amount of time taken since the last sweep started.
- More GC stats for better tweaking.

src/common/engine/stats.h

@@ -221,6 +221,11 @@ class cycle_t
 		Counter = 0;
 	}
 
+	void ResetAndClock()
+	{
+		Counter = -static_cast<int64_t>(rdtsc());
+	}
+
 	void Clock()
 	{
 		int64_t time = rdtsc();