83512d2 [Trivial] Mark CCoinsView child classes methods as override (random-zebra)
5643075 Store/allow tx metadata in all undo records (random-zebra)
8c4add4 Report on-disk size in gettxoutsetinfo (random-zebra)
6cb2989 Remove/ignore tx version in utxo and undo (random-zebra)
03b9724 Add specialization of SipHash for 256 + 32 bit data (Pieter Wuille)
e3cd496 Introduce CHashVerifier to hash read data (random-zebra)
10baa70 error() in disconnect for disk corruption, not inconsistency (random-zebra)
802f8c8 Add SizeEstimate to CDBBatch (Pieter Wuille)

Pull request description:

  This backports the commits listed as "Preparation" in bitcoin#10195.
  The rest will be included in a followup PR.

  Built on top of:
  - [x] #1772
  - [x] #1771
  - [x] #1793

  Here we:
  - add in-memory size estimation of a LevelDB batch
  - remove the usage of `error()` in ApplyTxInUndo/DisconnectBlock
  - Introduce the new CHashVerifier class to hash read data
  - Add a specialized version of SipHash for tuples of 256 bits and 32 bits data
  - Remove the transaction version from utxo and undo data structures
  - Add on-disk size of the utxo set to gettxoutsetinfo
  - store tx metadata in all undo records, instead of records corresponding only to the last output of a tx being spent.

ACKs for top commit:
  furszy:
    looking good, utACK 83512d2.
  Fuzzbawls:
    utACK 83512d2

Tree-SHA512: 7db730058ae208535aad5f5989abb9c55fd3722b79efef24fdd8c717687b67272bddaeae5e5a4a2d3e97626a7ec62a5d1ebdb5af0233f4a950b5a0c0b154484f

535d8e4 scripted-diff: various renames for per-utxo consistency (random-zebra)
60c73ad Rename CCoinsCacheEntry::coins to coin (Pieter Wuille)
1166f1f Merge CCoinsViewCache's GetOutputFor and AccessCoin (Pieter Wuille)
f25d3c6 [MOVEONLY] Move old CCoins class to txdb.cpp (random-zebra)
aab17b3 Upgrade from per-tx database to per-txout (Pieter Wuille)
a55eb98 Reduce reserved memory space for flushing (random-zebra)
7d637ea Remove unused CCoins methods (random-zebra)
4b7b1a3 Extend coins_tests (random-zebra)
8cf2dc0 Switch CCoinsView and chainstate db from per-txid to per-txout (random-zebra)
b20a662 [Cleanup] Remove unused CCoinsViewCache::IsOutputAvailable (random-zebra)
ea82855 Refactor GetUTXOStats in preparation for per-COutPoint iteration (Pieter Wuille)
0f9f2b6 [Core] Fix not-pruned-check in miner for zerocoin mint outputs (random-zebra)
3698d47 Replace CCoins-based CTxMemPool::pruneSpent with isSpent (Pieter Wuille)
11c728a Remove ModifyCoins/ModifyNewCoins (Pieter Wuille)
3c65936 Switch tests from ModifyCoins to AddCoin/SpendCoin (random-zebra)
b05fe55 Switch CScriptCheck to use Coin instead of CCoins (random-zebra)
9e5c2ae Only pass things committed to by tx's witness hash to CScriptCheck (random-zebra)
22b0b4a [Cleanup] fix warning in comparisons with nCoinbaseMaturity (random-zebra)
666081d Switch from per-tx to per-txout CCoinsViewCache methods in some places (random-zebra)
53fc2be [Core] PIVX: remove potential_overwrite: BIP34 has always been enforced (random-zebra)
16924aa Introduce new per-txout CCoinsViewCache functions (random-zebra)
2d74bc1 Optimization: Coin&& to ApplyTxInUndo (random-zebra)
3fcb092 Replace CTxInUndo with Coin (random-zebra)
42a8996 [Core] PIVX: Add fCoinStake boolean field to Coin class (random-zebra)
a01a9f2 Introduce Coin, a single unspent output (random-zebra)

Pull request description:

  The chainstate database, and its cache, rely on a per-tx model (i.e. it is a map from txids to *lists* of unspent outputs).
  This PR chages it to a per-txout model (i.e. a map from outpoints to *single* unspent outputs).

  Pros:
  - Simpler code.
  - Avoiding the CPU overhead of deserializing and serializing the unused outputs.
  - More predictable memory usage.
  - More easily adaptable to various cache flushing strategies.

  Cons:
  - Slightly larger on-disk representation, and sometimes larger in-memory representation (when there are multiple outputs for the same txid in the cache, which becomes optional).

  Adapted from bitcoin#10195 (adding a `fCoinStake` boolean memeber to the Coin class and considering BIP34 always in effect as per #1775)

  Based on top of:
  - [x] #1799
  - [x] #1800
  - [x] #1797
  - [x] #1796
  - [x] #1795
  - [x] #1793
  - [x] #1773

  The actual PR starts with "Introduce Coin, a single unspent output" (171bf0b8a55de0e3f94305b33e97bb1cf4a01b70)

  ⚠️  **Note for testers**: Do a backup of the chainstate before testing. With this PR, at startup, the database is automatically upgraded to the new format (so a reindex is not needed).

ACKs for top commit:
  furszy:
    Code review ACK 535d8e4. Need some live testing now.
  furszy:
    ACK 535d8e4
  Fuzzbawls:
    ACK 535d8e4

Tree-SHA512: 8648d31c343ff901f8568c14d0848241c9b8aca0b9a8f89f3e5dbe02d384ba35faf572757e84f979fba1f198d721a2516d0e784b5e0dc8d02b6e308b4278b484