composes: keep metadata where it won't be garbage collected
    
    This copies the metadata for nightly composes to a separate
    location which will not be garbage-collected, so we have a store
    of metadata from old composes now PDC is gone. It also writes
    the compose's compose ID into a "series" file, which makes it
    possible to sanely do things like "what was the compose before
    this compose?" without having PDC.
    
    The metadata for a full compose weighs about 150M. Almost all of
    this is rpms.json; withut that it's under 100K. If 150M per
    compose for eternity is too big, we could leave out rpms.json.
    
    The series file for Rawhide will grow eternally, but it's just a
    text file. We do one compose per day (usually), so in 100 years
    it'll be 36500 lines long, which doesn't really seem like a
    problem.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>