* Add PutMetadata function to storage backends
This function is not currently used, but it will be useful for helper
scripts that need to regenerate metadata on the fly, especially scripts
to migrate between storage backends. In the future, we can also use it
to automatically regenerate metadata if it is found to be missing or
corrupted.
* Add PutMetadata function to storage backend interface and
implementations
* Rework metadata generation to be more efficient and work better with
the PutMetadata function
* Add a basic test for metadata generation
* Change PutMetadata to take a Metadata type instead
It's unlikely that this function is useful if it always regenerates the
metadata. Instead, the caller should do that if it needs.
mutantmonkey