RFR: 8218746: SA: Implement discontiguous bitmap for ZGC

[Stefan Karlsson]

Hi all,

Please review this patch to implement a discontiuous bitmap for ZGC in 
the SA.

http://cr.openjdk.java.net/~stefank/8218746/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8218746

ZGC uses a 16TB virtual memory address range, so the normal scheme to 
map a bitmap over the entire address range uses too much memory. This 
patch lazily creates smaller bitmaps per ZPage, when at() or atPut() is 
called.

This patch builds upon JDK-8218743.

Tested with serviceability/sa and manually running Reverse Object 
Analysis in the SA.

Thanks,
StefanK