Refactor local client fs operations to new interface

[fryz]

Before this MR

The local bench client interweaved the data loading operations with the client code which exposed the Bench data objects to the higher-level business logic.

After this MR

There is a new local filesystem (fs) client which abstracts the data loading operations from the business logic. This allows for us to swap in separate data loading client implementation (eg: an S3 client, maybe in the future a dynamo or some other nosql client).