RFR: 8224167: Refactor PtrQueue completed buffer processing

[Kim Barrett]

> On May 21, 2019, at 4:35 AM, Aleksey Shipilev <shade at redhat.com> wrote:
> 
> On 5/20/19 11:15 PM, Kim Barrett wrote:
>> Webrev:
>> http://cr.openjdk.java.net/~kbarrett/8224167/open.00/
> 
> Shenandoah parts look good.

Thanks.

> They do not seem to affect performance in adverse way.

I expected this to be pretty much performance neutral; it avoids some
wasted effort, but the benefit is pretty tiny.

> Let me understand: SATBMarkQueue::handle_completed_buffer does reply either empty buffer (after full
> filtering/enqueueing), or semi-full buffer if filters were applied and it still decided not to
> enqueue it.

Correct.  I was hoping the description for the pure virtual in
PtrQueue was sufficiently clear.