S3Guard: Remove LocalMetadataStore

We use LocalMetadataStore MetadataStore implementation in S3Guard only for testing.
Inside it uses Guava's cache for storing metadatas. We try to mimic how dynamo should work under the hood, but with every new feature or API modification what we do on MetadataStore interface level it gets more and more complicated to implement the same feature with different behavior.

I want to start a debate on why we need to remove that, or why we want to keep it.
I could rant about why is it annoying to have this implementation when we need to get some things right to work with dynamo, and then do a totally different set of modification in the LocalMetadata to get the same outcome, and also add more tests just for the thing we use for testing.

There are also areas in our ever-growing testing matrix that would need more attention instead of fixing tests for our test implementation. But on the other hand, it is good that we have another impl for the API which we can use for drafting new ideas.