Improve reliability of KV Store API tests

[kpfleming]

Writing and then reading an item in the KV Store can sometimes fail, as the newly-written content has to replace the existing content in the cache and that does not happen instantaneously.

This PR adds logic to loop for up to 3 seconds in the KV Store tests to ensure that the newly-written content can be read.

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[kpfleming]

This is blocked until we can upgrade to the latest version of go-vcr, and we can't do that until after #626 has been merged.