When adding a new target, it should clone all data from the existing target

[ericflo]

Right now the storage providers don't like being in disagreement, and adding a new target directory to an existing installation means that they will inherently disagree, with no real recourse currently. Since this use case is likely, we should make sure that the new storage instance's view of the world matches that of the existing set before we add it.

In practice, this means adding one or two more methods to the storage.Storage interface: one to read all contents of the database, and one to ingest that data. And then making sure those functions are called by add-target.