RFR: Resettable iterators to avoid dealing with copying/assignment compilation differences

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/iterator-copy-reset/webrev.01/

Following up on MacOS/Clang build failures. The way we deal with iterators is still not good, and
Clang complains that it wanted to use copying constructor -- and failed, because we marked it
private. Let's instead disallow copying and make explicit resets where needed.

Testing: hotspot_gc_shenandoah, eyeballing gc cycle

Thanks,
-Aleksey