RFR: Adjust metaspace size during each GC

[Zhengyu Gu]

Adjust metaspace size during each GC cycle.

This patch, on top of early "Handle metadata induced GC" patch, clears 
vmTestbase/metaspace tests


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

Test:
   tier3_gc_shenandoah (fastdebug and release)
   vmTestbase/metaspace (fastdebug and release)

Thanks,

-Zhengyu