CDK-353: Add TemporaryDatasetRepository for MR.

This updates MR to use datasets from a temporary repository rather than
datasets in the target repository for temporary MR records and files.
Otherwise, jobs writing to Hive will create a dataset for each reduce
task attempt, which all send partition updates.

This introduces a TemporaryDatasetRepository and a
TemporaryDatasetRepositoryAccessor so that repository implementations
can supply a more appropriate repository. FS datasets return
repositories rooted in .temp/ underneath the repository's root
directory, and Hive's temporary repository does not interact with Hive.