RFR: 8221153: ZGC: Heap iteration and verification pollutes GC statistics

[Per Liden]

When performing heap iteration and verification we should disable 
ZStatTimers to avoid polluting the GC statistics. This can otherwise 
look very odd in GC logs, with statistics that is hard to interpret 
since the times recorded doesn't come from a GC pause.

This patch introduces ZStatTimerDisable, which can be placed in scopes 
where we want ZStatTimers to be disabled.

Bug: https://bugs.openjdk.java.net/browse/JDK-8221153
Webrev: http://cr.openjdk.java.net/~pliden/8221153/webrev.0

/Per