Restore the definition of HHWheelTimer::UniquePtr

Restore the definition of HHWheelTimer::UniquePtr

Summary: Changing the definition of `HHWheelTimer::UniquePtr` wasn't safe, because some clients were using that type outside of the `HHWheelTimer::newTimer` helper. I'm changing that part back. We'll still be able to proceed with my other codemod to `HHWheelTimer`, but we'll always have two different smart pointer types to manage: `UniquePtr` and `IntrusivePtr`.

Reviewed By: djwatson

Differential Revision: D2644721

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