RFR: Cleanup header files and forward declarations

[Zhengyu Gu]

A round of cleanup: header files (and orders) and forward declarations, 
follows hotspot convention that import header file as needed.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/cleanup_headers/webrev.00/

Test:

   PHC and non-PHC builds

   tier2_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu