[16] RFR(S): 8247218: Add default constructor to VectorSet to use Thread::current()->resource_area() as arena by default

[Christian Hagedorn]

Hi

Please review the following cleanup:
https://bugs.openjdk.java.net/browse/JDK-8247218
http://cr.openjdk.java.net/~chagedorn/8247218/webrev.00/

By adding an additional default constructor in VectorSet (as also 
present in Node_List, Node_Stack etc.), we can replace the often found 
pattern

   Arena* a = Thread::current()->resource_area();
   VectorSet visited(a);

by a simple

   VectorSet visited;

I additionally cleaned up some other places where this pattern was 
found, for example for Node_List etc.

Thank you!

Best regards,
Christian