[Consumer] Issue warning when discarding a document with identical sequence number and different content

[EyalLavi]

When the consumer receives a document with a pair of sequence identifier and sequence number that is identical to an already seen pair, the document is discarded: #258, R107. It will be nice to also issue a warning message if the content would otherwise be different from the one we already have in the sequence object. For equality testing, use the same 'deep-equal' method as the buffer delay node (i.e. passive node). This separates out part of #258.