Use Varint serialization

[kcalvinalvin]

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Things done:

1. Commit accumulator: Use varints for serialization imports the wire package and uses the varint serialization functions for batchproof.
2. Commit accumulator/undo: Add serialization for undoblocks adds undoblock serialization/deserialization (uses varint) and exports UndoBlockto expose the serialization functions.
3. Commit go.mod, go.sum: Switch to the btcd++ branch in utcd now imports utcd instead of btcd. However, it only imports the btcd++ branch as that branch doesn't have any utreexo specific packages, therefore not creating any dependency cycles.

This PR doesn't produce backwards compatible proofs as the serialization is different.