ReliableFIFO: Serialize JSON to binary octets instead of unicode codepoints.

1.  The JSON serializer now reads and writes serialized data as binary
    octets instead of unicode endpoints. This is necesary because the
    Redis module sends and receives binary octets.
    
    This fixes a bug where items with certain special characters won't be
    removed from the busy queue in Queue::Q::ReliableFIFO::Redis because
    Redis.pm returns binary octets while Queue::Q::ReliableFIFO::Item
    expects Unicode codepoints, causing eventual double encoding.

2. Throw a warning when items are not properly removed from the 'busy'
    queue when using the Chunk => 1 option (which is the default one).