Make collections JSON cache expire after a day
    
    I tried to implement this in fedora_nightlies last year but
    found recently that it wasn't working. I just figured out that
    is because fedora_nightlies was using get_current_release, which
    uses this dumb cache, so even though I had a smart cache with
    expiry in fedora_nightlies, when that cache expired and was
    refreshed it just hit *this* dumb cache and got old data anyway.
    So let's make *this* cache smart, then I can drop the caching
    from fedora_nightlies entirely, and other things that use the
    fedfind helpers that run off this collections JSON - like the
    update job scheduler - won't be working off overly stale data
    either.
    
    This adds a new test dependency on freezegun, as we need to
    MANIPULATE TIME (MUAHAHAHAHA) to test this.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>