RFR: 8211718: Supporting multiple concurrent OopStorage iterators

[Erik Österlund]

Hi,

OopStorage has an artificial restriction of having only one concurrent 
iterator at a time. This restriction should be removed.

The solution is to turn the bool that tracks concurrent iterations, into 
an int.

For some background, the reason for the restriction has historic 
reasons, from a time when opening OopStorage for concurrent iteration 
was done using a lock-free protocol between GC worker threads, which 
made it tricky. Since then, the model was simplified to just let the 
ParState RAII object open and close the concurrent iterations. With the 
simplified model, there is no need for such a restriction.

I talked to Kim, and long-term there are more plans in this area, such 
as removing weak_oops_do, and then unifying the iterators. But for now, 
I would like to just stop disallowing multiple concurrent iterations as 
one small step along the way.

I updated the relevant OopStorage documentation to reflect the removal 
of this restriction.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8211718/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8211718

Thanks,
/Erik
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