RFR(S): 8227225: ParallelGC: add subspace transitions for young gen for gc+heap=info log lines

[Kim Barrett]

> On Aug 6, 2019, at 3:49 PM, Tony Printezis <tprintezis at twitter.com> wrote:
> 
> So, CMS is apparently going away. Is Serial going away too? Is there need for a similar change for GenCollectedHeap (which will work for both GCs)?

I’ve not heard of any proposal to make Serial go away.
There might be some post-CMS-removal cleanup, since it would be the only GenCollectedHeap.
But I don’t think adding subspace transition reporting should wait for that.