hazptr: Replace friend "swap" with a member function and a non-friend

hazptr: Replace friend "swap" with a member function and a non-friend

Summary:
This matches what the STL does with e.g. std::vector::swap() and std::shared_ptr::swap().

http://en.cppreference.com/w/cpp/container/vector/swap
http://en.cppreference.com/w/cpp/memory/shared_ptr/swap

Should be relatively uncontroversial, I would think.
Closes https://github.com/facebook/folly/pull/489

Reviewed By: magedm

Differential Revision: D3963285

Pulled By: yfeldblum

fbshipit-source-id: 3fa6bf77e66fa9a673cb648b0bf87b1db3caa6c8