bitcoin

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merge-script c90c23d388 Merge bitcoin/bitcoin#35531 : txindex: hash keys and pack positions to reduce disk usage

25bed560be test: add forward-compat functional test for txindex (sedited)
703304ed8c doc: add release notes for txindex disk usage and downgrading (Andrew Toth)
8e5320a2d2 tests: cover txindex hash prefix collisions and legacy fallback (Andrew Toth)
b75efa19ba txindex: skip bloom filters and legacy lookups for new databases (Andrew Toth)
004d7c098c txindex: hash key prefixes and pack block positions (Andrew Toth)
5a255970fd refactor: move txindex db constants and legacy key to txindex_key.h (Andrew Toth)
327660134c txindex: pass the full block to DB::WriteTxs (Andrew Toth)
42771e7998 txindex: use a new block locator for downgrade safety (Andrew Toth)
4b08baed72 txindex: return optional tx and block hash from FindTx (Andrew Toth)

Pull request description:

  The current txindex uses the full 32-byte txid as keys, which takes up about 66 GB of disk space today on mainnet. Using a 5-byte key prefix instead drops the disk usage to 26 GB - cutting the size to less than half.

  Using the full 32-bytes is unnecessary since a 5-byte salted siphash will produce collisions in about 1 in 1.1 trillion. Some collisions will occur, but the penalty is just an extra disk read, deserialization and hash.
  The tx position can be appended to the key instead of used as a value, and a LevelDB iterator can seek to the prefix and then scan for the correct tx. This is an almost identical approach to `txospenderindex`.

  Also instead of storing the file position of the block, we can store only the sequence of the connected block and offset of the transaction in the block. This can be packed into a 6-byte key suffix using 3-byte representations of the sequence and offset in the block. The block file can be recovered by the CBlockIndex that is already in memory. The sequence is mapped to the block hash in the db, so we can lookup the block hash to find the CBlockIndex during reads.

  If a tx is not found with this method, we fallback to looking up the legacy entry. With this method a user with an existing db can opt to erase the `indexes/txindex` folder and reindex, or keep the current index and new entries will be appended with the smaller footprint.

  The time to index was faster on my machine with this method, 1h19m vs current 1h50m.
  Lookups are roughly the same, around 0.2ms per lookup with `getrawtransaction`.
  When testing on mainnet, I got 894,549 2-way collisions, 395 3-way collision, and 1 4-way collision that worst case could cause an extra 3 false positives when reading.

ACKs for top commit:
  l0rinc:
    diff reACK 25bed560be
  sedited:
    ACK 25bed560be
  ajtowns:
    ACK 25bed560be

Tree-SHA512: a25c79ca7e722e2f372b65f5fc11c8b194ad49f2240b4881c7e606306aabbd3604aede3f1c33606b467486affac3a3f503638f513c896935cebbc02709cb60d8