Refactor PtrQueue completed buffer processing

The code path for handling a completed buffer starts in handle_zero_index. The current path involves functions and configuration checks that are only applicable for SATB queues, and other functions and configuration checks that are only applicable for G1 dirty card queues. In each case, the inapplicable stuff clutters things, making it harder to understand and wasting time.

Refactor to streamline the shared PtrQueue code and move the subclass-specific stuff into the subclasses.