This method is the allocation entry API for mutators. Its flow is the following

1. try young-gen
2. try old-gen
3. invoke GC operation
4. if GC-op succeeds, return; otherwise, go to 1

Before step 3, we need to call `total_collections()`, which requires holding `Heap_lock`. The current code invokes old-gen allocation while holding `Heap_lock`.

However, the old-gen allocation API supports multi-thread, so it can be moved outside the critical-region. This cleans up the structure and streamlines the flow.