Set: Avoid using hazard pointers for the lifetime of an iterator

[jleben]

Since an iterator uses a number of hazard pointers throughout its lifetime, and it is possible that the user has multiple iterators at the same time (by nesting iterations), the number of hazard pointers potentially used by a thread is not statically bounded.

Can we use hazard pointers only during execution of iterator methods, instead of throughout an iterator's lifetime?