RFR: 8231552: ZGC: Refine address space reservation

[Per Liden]

Latest version of this patch, rebased on today's jdk/jdk:

http://cr.openjdk.java.net/~pliden/8231552/webrev.2

/Per

On 10/3/19 11:45 AM, Per Liden wrote:
> We could be slightly more sophisticated and do a better job reserving 
> address space in situations where parts of the address space is already 
> occupied or when the process is running with address space limitations.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8231552
> Webrev: http://cr.openjdk.java.net/~pliden/8231552/webrev.0
> 
> /Per