change the serialization/unserialization code, mirroring CTxInUndo class
(which will be removed in next commit, and replaced with a serialization
adapter for Coin)

>>> backports bitcoin/bitcoin@cb2c7fd

The earlier CTxInUndo class now holds the same information as the Coin
class. Instead of duplicating functionality, replace CTxInUndo with a
serialization adapter for Coin.

>>> backports bitcoin/bitcoin@bd83111

This avoids a prevector copy in ApplyTxInUndo.

>>> backports bitcoin/bitcoin@0003911

The new functions are:
* CCoinsViewCache::AddCoin: Add a single COutPoint/Coin pair.
* CCoinsViewCache::SpendCoin: Remove a single COutPoint.
* AddCoins: utility function that invokes CCoinsViewCache::AddCoin for
  each output in a CTransaction.
* AccessByTxid: utility function that searches for any output with
  a given txid.
* CCoinsViewCache::AccessCoin: retrieve the Coin for a COutPoint.
* CCoinsViewCache::HaveCoins: check whether a non-empty Coin exists
  for a given COutPoint.

The AddCoin and SpendCoin methods will eventually replace ModifyCoins
and ModifyNewCoins, AddCoins will replace CCoins::FromTx, and the new
AccessCoins and HaveCoins functions will replace their per-txid
counterparts.

Note that AccessCoin for now returns a copy of the Coin object. In a
later commit it will be change to returning a const reference (which
keeps working in all call sites).

ref: PIVX-Project#1775

>>> backports bitcoin/bitcoin@f68cdfe

>>> backports bitcoin/bitcoin@c87b957

This clarifies a bit more the ways in which the new script execution
cache could break consensus in the future if additional data from
the CCoins object were to be used as a part of script execution.

After this change, any such consensus breaks should be very visible
to reviewers, hopefully ensuring no such changes can be made.

>>> backports bitcoin/bitcoin@8b3868c

>>> adapts bitcoin/bitcoin@961e483

>>> backports bitcoin/bitcoin@5083079

This patch makes several related changes:
* Changes the CCoinsView virtual methods (GetCoins, HaveCoins, ...)
  to be COutPoint/Coin-based rather than txid/CCoins-based.
* Changes the chainstate db to a new incompatible format that is also
  COutPoint/Coin based.
* Implements reconstruction code for hash_serialized_2.
* Adapts the coins_tests unit tests (thanks to Russell Yanofsky).

A side effect of the new CCoinsView model is that we can no longer
use the (unreliable) test for transaction outputs in the UTXO set
to determine whether we already have a particular transaction.

>>> backports bitcoin/bitcoin@ce23efa

>>> backports bitcoin/bitcoin@97072d6

>>> backports bitcoin/bitcoin@b2af357

As the maximum amount of data that can be pulled into the cache due to
a block validation is much lower now (at most one CCoin entry per input
and per output), reduce the conservative estimate used to determine
flushing time.

>>> backports bitcoin/bitcoin@580b023

It's only used for upgrading from the old database anymore.

They're doing the same thing now.

>>> backports bitcoin/bitcoin@5898279

Thanks to John Newberry for pointing these out.

-BEGIN VERIFY SCRIPT-
sed -i 's/\<GetCoins\>/GetCoin/g' src/*.cpp src/*.h src/*/*.cpp
src/*/*.h
sed -i 's/\<HaveCoins\>/HaveCoin/g' src/*.cpp src/*.h src/*/*.cpp
src/*/*.h
sed -i 's/\<HaveCoinsInCache\>/HaveCoinInCache/g' src/*.cpp src/*.h
src/*/*.cpp src/*/*.h
sed -i 's/\<IsPruned\>/IsSpent/g' src/*.cpp src/*.h src/*/*.cpp
src/*/*.h
sed -i 's/\<FetchCoins\>/FetchCoin/g' src/*.cpp src/*.h src/*/*.cpp
src/*/*.h
sed -i 's/\<CoinsEntry\>/CoinEntry/g' src/*.cpp src/*.h src/*/*.cpp
src/*/*.h
sed -i 's/\<vHashTxnToUncache\>/coins_to_uncache/g' src/*.cpp src/*.h
src/*/*.cpp src/*/*.h
sed -i 's/\<vHashTxToUncache\>/coins_to_uncache/g' src/*.cpp src/*.h
src/*/*.cpp src/*/*.h
sed -i 's/\<fHadTxInCache\>/had_coin_in_cache/g' src/*.cpp src/*.h
src/*/*.cpp src/*/*.h
sed -i 's/\<coinbaseids\>/coinbase_coins/g' src/*.cpp src/*.h
src/*/*.cpp src/*/*.h
sed -i 's/\<disconnectedids\>/disconnected_coins/g' src/*.cpp src/*.h
src/*/*.cpp src/*/*.h
sed -i 's/\<duplicateids\>/duplicate_coins/g' src/*.cpp src/*.h
src/*/*.cpp src/*/*.h
sed -i 's/\<oldcoins\>/old_coin/g' src/test/coins_tests.cpp
sed -i 's/\<origcoins\>/orig_coin/g' src/*.cpp src/*.h src/*/*.cpp
src/*/*.h
-END VERIFY SCRIPT-