Requirements to support node&wallet

[kcalvinalvin]

For a node/wallet to used the rustreexo library, we need to support additional things in the current rustreexo library.

• add() method on stump needs to return new hashes and their positions
• del() method on stump needs to return new hashes in their current positions
• Need to support add() on cached proofs
• Need to support del() on cached proofs
• Extra: Need to support undo on cached proofs

Additionally, the node either needs to be able to parse utreexo block messages or the utreexod node in github.com/utreexo/utreexod needs to support an rpc call that returns utreexo proofs for a requested block.