Use the Coin abstraction to undo spends.

Summary:
This is useful during reorg. It is based on work by Pieter Wuille and a continuation of D342 .

It include special serialization and deserialization routines to ensure backward compatibility with existing on disk undo data.

Test Plan:

make check
../qa/pull-tester/rpc-test.py

Did a 5000 block deep reorg on testnet using invalidateblock/reconsiderblock

Reviewers: freetrader, kyuupichan, sickpig, awemany, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D368