Handle refcounting when clearing SortEx buffer.

Now that SortExternal only handles objects rather than addresses, it
makes sense to decref objects when emptying out the buffer.

This change is safe for all subclasses and incurs no performance penalty
under normal conditions -- unless something goes wrong all buffered
items will have already been consumed by the time the buffer-clearing
operation is invoked.
Move Shrink() up from PostingPool to SortExternal.

Move the Shrink method up into the parent class and generalize it by
calling Shrink() on runs.
Purge some obsolete code from SortExternal.