Batch value update API on mutable in-memory provider

Could be useful to allow making one larger update and for better testing of code relying on atomic update of multiple values.

Start by locating the mutable in-memory provider and its existing value-update API. Compare current update operations and tests to determine the expected batch shape and atomicity. Done means the provider exposes a batch update that changes multiple values as one operation and supports testing code that relies on that behavior.

Written by the indexing model from the issue text.