RFR: 8213623: ZGC: Let heap iteration walk all roots

[Per Liden]

Let the ZHeapIterator walk all roots, including weak roots that might be 
logically cleared from the application's point of view. This allows e.g. 
for a more thorough heap verification.

Also the heap iterator should access oops with AS_NO_KEEPALIVE to avoid 
resurrecting object that would otherwise be garbage collected.

Bug: https://bugs.openjdk.java.net/browse/JDK-8213623
Webrev: http://cr.openjdk.java.net/~pliden/8213623/webrev.0

Testing: tier1-4

/Per