Add CTxUndo: transaction undo information
Add CTxUndo: transaction undo information

The CTxUndo class encapsulates data necessary to undo the effects of
a transaction on the txout set, namely the previous outputs consumed
by it (script + amount), and potentially transaction meta-data when
it is spent entirely.

wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin