Remove duplicate manifest file record metadata

[jwakre]

When a file is referenced in multiple manifests, multiple identical copies of the file's metadata will exist (hash, name, stat() info). The memory usage could be reduced by storing only one copy of a file's metadata and referencing it multiple times.

The File struct in files.go contains both file and manifest metadata. By separating the file and manifest metadata into 2 structs, the file metadata collected once and referenced multiple times.