RFR(M): 8204613: StringTable: Calculates wrong number of uncleaned items.

[Robbin Ehn]

Hi all, please review.

The StringTable lazy evicts dead string, until a dead string is evicted it will 
be counted as a dead string. If it is not evicted before next GC cycle it is 
counted again, making the count of uncleaned strings skew.
Also ZGC walks the strings without using the stringtable GC API, but it needs to 
be-able to feedback the number of dead strings to get the cleaning functionality.
There is a big probability that ZGC makes it in before this change-set, so I 
included ZGC changes.

There was a compile issue on slowdebug on windows for create_archived_string(), 
I added NOT_CDS_JAVA_HEAP_RETURN_(NULL) for it.

Change-set: http://cr.openjdk.java.net/~rehn/8204613/webrev/index.html
Bug: https://bugs.openjdk.java.net/browse/JDK-8204613

T1-3 with ZGC testing on, no related issues and manual JMH testing.

Thanks, Robbin