Support starting a transaction directly against a partition

Storage interface contains a Begin method that can currently be used to start a transaction against a specified partition. This is needlessly complex as it makes the Begin method have multiple modes by either starting a transaction against a specified partition or from looking up a relative path's partition assignment. We'd ideally only have a Begin method on the partition layer as that's where the transactions ultimately belong.

This MR takes a small clean up step by implementing a GetPartition method that can be used to directly access a partition by its ID. The users of Begin with partition ID are updated to use that instead.