RFR: 8210061: ZGC: Remove STW weak processor mode

[Robbin Ehn]

Hi Erik, looks good!

I didn't like that I had to add ZConcurrentStringTable and it required more 
testing, thanks for removing it!

/Robbin

On 08/28/2018 11:58 AM, Erik Österlund wrote:
> Hi,
> 
> ZGC has multiple modes for processing weaks. ZWeakRoots allows treating weaks as 
> strong, and in addition to that, ZConcurrentStringTable, 
> ZConcurrentVMWeakHandles and ZConcurrentJNIWeakGlobalHandles allow certain 
> subsets of weak roots to be processed concurrently or in a pause. Except there 
> is no obvious reason why you would ever opt out of doing it concurrently. So I 
> propose to remove this functionality.
> Part of the motivation is that a concurrent strong root processor is on its way, 
> and all these modes nobody uses leads to pointless complexity.
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8210061/webrev.00/
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8210061
> 
> Thanks,
> /Erik