* Replace UnsafeCell in DateServiceInner with Cell
The previous API was extremely dangerous - calling `get_ref()`
followed by `reset()` would trigger instant UB, without requiring
any `unsafe` blocks in the caller.
By making DateInner `Copy`, we can use a normal `Cell` instead
of an `UnsafeCell`. This makes it impossible to cause UB (or even panic)
with the API.
* Split unsafe block HttpServiceHandlerResponse
Also add explanation of the safety of the usage of `unsafe`
* Replace UnsafeCell with RefCell in PayloadRef
This ensures that a mistake in the usage of 'get_mut' will cause
a panic, not undefined behavior.
