RFR (M) 8235573: Move JFR ObjectSample oop into OopStorage

[Kim Barrett]

> On Aug 3, 2020, at 9:14 AM, coleen.phillimore at oracle.com wrote:
> On 8/1/20 5:14 AM, Kim Barrett wrote:
>> One less serial weak processing phase!  Yay!
> 
> We thought you'd like that.  Unfortunately, the jvmti serial phase is hard to eradicate.

I had an idea for that a long time ago, but haven’t followed up.

> incremental webrev at http://cr.openjdk.java.net/~coleenp/2020/8235573.02.incr/webrev
> full webrev at http://cr.openjdk.java.net/~coleenp/2020/8235573.03/webrev

_last_sweep is being written by one thread and read others. For STW
reference processing collectors this might not be a problem; I'm not sure
what threads do the reading and what their state is when the read is
performed. But even if that's the case, this seems like it should a be a
problem for concurrent reference processing.

> And retested.
> Thanks,
> Coleen