Fix mtime/ctime checksum type

The mtime/ctime checksum types convey the source ctime/mtime from the server and compare it against the local file on disk. Since it does straight equality, this will always fail.

This needs to change to a reasonable comparison, either by making mtime/ctime a property that's updated on the target, or only updating if mtime/ctime on the source is newer than mtime/ctime on the target.