RFR 8026985: Rewrite SystemDictionary::classes_do and Dictionary::classes_do to use KlassClosure

[coleen.phillimore at oracle.com]

Summary: Clean up and examine uses of classes_do for the SystemDictionary

See bug comments for more details.  I wanted to clean this up while 
examining the idea of having system dictionary information added per 
ClassLoaderData, rather than a global table.

open webrev at http://cr.openjdk.java.net/~coleenp/8026985.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8026985

Tested with java.lang.instrument, sun.com.jdi, tonga colocated (closed) 
tests, and JPRT, because of difference in which classes_do is called for 
heap dumping.

Note, will update copyrights on commit.

Thanks,
Coleen