Summary:
Updated to the interface to be in synch with the latest standard proposal in P0233R4 as follows:
- Renamed hazptr_owner as hazptr_holder.
- Combined hazptr_holder member functions set() and clear() as reset().
- Replaced the template parameter A for hazptr_holder member function templates try_protect() and get_protected with atomic<T*>.
- Moved the template parameter T from the class hazptr_holder to its member functions try_protect(), get_protected(), and reset().
- Added a non-template overload of hazptr_holder::reset() with an optional nullptr_t parameter.
- Removed the template parameter T from the free function swap() as hazptr_holder is no longer a template.

Reviewed By: davidtgoldblatt

Differential Revision: D5283863

fbshipit-source-id: 2bc1a09f4f844aa72d9b7dff9c450540bbe09972