RFR: 8151693: Move print_heap_before/after_gc to debug level

[Stefan Johansson]

Hi,

Please review this small change for RFE:
https://bugs.openjdk.java.net/browse/JDK-8151693

Webrev:
http://cr.openjdk.java.net/~sjohanss/8151693/hotspot.00/

Summary:
Previously the before and after heap info logging was on trace level. 
With this change is has been moved to debug level to allow it to be 
printed without getting to much other info. I've run the gc jtreg tests 
as sanity checking and the tests pass.

Thanks,
Stefan