Add support for x-amz-metadata-directive in S3 and native backend

[ChrisSchinnerl]

Right now we always preserve user metadata attached to objects but S3 also supports the "REPLACE" behavior via the x-amz-metadata-directive header.

To support that, we would first need to update gofakes3 and pass that option on to the relevant interface methods. Then we can extend renterd to make use of that.