CORE: Introduce SimplifiedWitnessUTXOSet and SimplifiedWitnessRouletteItem
CORE: Introduce SimplifiedWitnessUTXOSet and SimplifiedWitnessRouletteItem

To be used to encapsulate the bare minimum information required to perform witness selection; without all the additional "book keeping" info needed for other parts of the system.
This will be used for SPV witness sync.

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