Remove get and set from AtomicCache

[trowski]

I believe these methods were originally added with the thought this class should also implement Cache, but it doesn't, and as an atomic cache, it likely shouldn't provide methods that could be a footgun.

[kelunik]

Where do you see footgun potential?

[trowski]

get and set increase the potential for inadvertently breaking atomicity of the cache by setting a value based on a prior get. get by itself may be acceptable and can perhaps remain, but providing set makes it too easy to break the contract and overwrite a cache value based on a stale value.