Define Transaction interface in storage package

There was previously a cyclic dependency between the 'git' and 'storage' packages. We've solved this now by splitting the implementation related bits out from the 'git' package and only type definitions and interfaces there. With the cycle out of the way, we can now define the Transaction interface in a single location and reuse it across all the sites that previously were using one of the other interface definitions.

This MR defines the interface of a Transaction in the storage package. We also replace duplicate definitions of the interface in other packages.