Make PolledServiceHealth::ServiceHealth 1 byte
Make PolledServiceHealth::ServiceHealth 1 byte

Summary: This allows us to use the other bits of `std::atomic<int64_t>` to store additional metadata like a timestamp. See D31256652.

Reviewed By: yfeldblum

Differential Revision: D31256650

fbshipit-source-id: 2c886299caca34fd5205e72d000531b84c66cfdd

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