Add integration test version of TestMetadataStore for DynamoDB

I tweaked TestDynamoDBMetadataStore to run against the actual Amazon DynamoDB service (as opposed to the "local" edition). Several tests failed because of minor variations in behavior. I think the differences that are clearly possible are enough to warrant extending that class as an ITest (but obviously keeping the existing test so 99% of the the coverage remains even when not configured for actual DynamoDB usage).