Explore Validity of AC Store Comments in Store Trait

The CAS contractually has the size encoded in it so it's easy to know, the Action Cache does not have this. It's the size of the command not the size of the item.

Inclination is that the size is valid, however it should be verified.

The two 'Note' comments are the ones in question in store_trait.rs:

/// Look up a digest in the store and return None if it does not exist in
    /// the store, or Some(size) if it does.
    /// Note: On an AC store the size will be incorrect and should not be used!
    #[inline]
    async fn has(self: Pin<&Self>, digest: DigestInfo) -> Result<Option<usize>, Error> {
        let mut result = [None];
        self.has_with_results(&[digest], &mut result).await?;
        Ok(result[0])
    }

    /// Look up a list of digests in the store and return a result for each in
    /// the same order as input.  The result will either be None if it does not
    /// exist in the store, or Some(size) if it does.
    /// Note: On an AC store the size will be incorrect and should not be used!
    #[inline]
    async fn has_many(self: Pin<&Self>, digests: &[DigestInfo]) -> Result<Vec<Option<usize>>, Error> {
        let mut results = vec![None; digests.len()];
        self.has_with_results(digests, &mut results).await?;
        Ok(results)
    }

Start in store_trait.rs with has and has_many, then trace has_with_results through the Action Cache and CAS store implementations to compare the returned size with each digest's meaning. The work is done when the size behavior is established and the two Note comments accurately describe whether the value can be used.