Magic-folder uploads/downloads do not preserve filesystem metadata

[cypher]

Unlike the tahoe backup command, magic-folder uploads/downloads do not preserve filesystem metadata (such as mtime/ctime, uid/gid, file attributes, etc.); when Bob's shared magic-folder downloads a file uploaded by Alice, that file's mtime/ctime will be set to the time at which the download completed rather than the (arguably more desirable) times at which that file was originally modified/created on Alice's computer.

Preserving this metadata would be desirable not only for traditional archival purposes (since, e.g., the date that users may have taken certain pictures or written certain documents is often important) but also for the purposes of aiding users to resolve conflicts produced by magic-folder itself in a collaborative scenario (since, e.g., with the current implementation, Bob has no straightforward way of knowing whether the conflicting file produced on his computer by Alice was the one she edited last week or this morning).