RFR: 8150676: Use BufferNode index

[Kim Barrett]

Please review this change to PtrQueue and its derivatives to maintain
the index in BufferNode where needed.  This allowed the removal of
code to fill inactive leading portions of buffers with NULL and to
remove code to skip over NULL entries.

Removed unused DirtyCardQueueSet::apply_closure_to_all_completed_buffers, 
rather than fixing it's BufferNode manipulation.

Further changed SATBMarkQueue::filter to use two-fingered compaction,
which may further reduce the number of writes to the buffer during
filtering.  For example, using specjbb2015, with over 2.5M buffers
processed, the number of writes using the new two-fingered compaction
(12M) was a factor of 50 fewer than needed by the (non-NULLing)
sliding algorithm (60M), and a factor of 250 fewer than the original
sliding algorithm (330M).  On average, filtering a buffer removed
about 75% of the entries in that test.

CR:
https://bugs.openjdk.java.net/browse/JDK-8150676

Webrev:
http://cr.openjdk.java.net/~kbarrett/8150676/webrev.00

Testing:
JPRT
Aurora ad-hoc defaults + GC nightly + Runtime nightly