RFR: 8234543: ZGC: Parallel pre-touch

[Per Liden]

When using -XX:+AlwaysPreTouch, ZGC is currently doing single threaded 
pre-touch. This patch makes this a parallel operation. This improves 
startup time, especially when using large heaps. For example, when using 
a 3TB heap (-XX:+AlwaysPreTouch -Xms3T -Xmx3T), startup time is improved 
by about 30x.

Bug: https://bugs.openjdk.java.net/browse/JDK-8234543
Webrev: http://cr.openjdk.java.net/~pliden/8234543/webrev.0

/Per