10 RFR: 8175221: Cleanup DirtyCardQueueSet::concatenate_log

[Kim Barrett]

Please review this simplification of concatenate_log and removal of
some thereby unused public functions from DirtyCardQueue.

We change concatenate_log to call flush, rather than inlining more or
less equivalent code.  The flush is conditional on the queue
containing any data, as we prefer to leave an empty buffer in place in
the queue when concatenating.

We also changed flush to support this; flush was a nop when the queue
is "permanent", which is not what we want for concatenate_log.  This
behavior was because flush was called by the queue destructor, and
performing a flush when destroying the shared queue for a set was
problematic.  However, JDK-8048949 changed things so that only the
DirtyCardQueue destructor called flush, and only if the queue is
non-permanent.  So the permanent check in flush is no longer needed,
and removing it makes it suitable for use by concatenate_log.

CR:
https://bugs.openjdk.java.net/browse/JDK-8175221

Webrev:
http://cr.openjdk.java.net/~kbarrett/8175221/hotspot.00/

Testing:
jprt