Isolate low-level dataset storage repositories in a new crate and name key traits to emphasize STORAGE aspect

[zaychenko-sergei]

The repositories structure we are talking about are:

• main repository: DatasetRepository / DatasetRepositoryLocalFs / DatasetRepositoryS3
• Dataset, MetadataChain interfaces and implementations
• lower level ObjectRepository, NamedObjectRepository, and similar
• auxillary code around them

The key motivations are:

• we want a flexibility to construct and recompose these objects in both core and dataset domains
• we are thinking of a separate open source library to be able to write pure ODF datasets to disk or S3

The naming should emphasize "storage", so that it's easier to distinguish it from higher lveel DatasetRegistry.