implement a variable delay node

[malikbeytrison]

[EyalLavi]

The delay node has been split into Buffer and Retiming delay nodes. Both only accept fixed delays so I'm closing this issue.

[nigelmegitt]

Future work to make the delay node have variable delays is still legitimate, but does not necessarily need to be in R3. Reopening.

[EyalLavi]

Added 'buffer variable delay node' and 'retiming variable delay node' to the component list.

[EyalLavi]

Moved to R3 since this is how we verify the spec requirement that a delay node must not change the order of documents in a sequence.