Remove/ignore tx version in utxo and undo

This makes the following changes:
* In undo data and the chainstate database, the transaction nVersion
  field is removed from the data structures, always written as 0, and
  ignored when reading.
* The definition of hash_serialized in gettxoutsetinfo is changed to no
  longer incude the nVersion field. It is renamed to hash_serialized_2
  to avoid confusion. The new definition also includes transaction
  height and coinbase information, as this information was missing
  before.

This depends on having a CHashVerifier-based undo data checksum
verifier.

Apart from changing the definition of serialized_hash, downgrading
after using this patch is supported, as no release ever used the value
of nVersion field in UTXO entries.

doc/release-notes.md

@@ -36,6 +36,15 @@ Notable changes
 Low-level RPC changes
 ---------------------
 
+- The new database model no longer stores information about transaction
+  versions of unspent outputs. This means that:
+  - The `gettxout` RPC no longer has a `version` field in the response.
+  - The `gettxoutsetinfo` RPC reports `hash_serialized_2` instead of `hash_serialized`,
+    which does not commit to the transaction versions of unspent outputs, but does
+    commit to the height and coinbase information.
+  - The `getutxos` REST path no longer reports the `txvers` field in JSON format,
+    and always reports 0 for transaction versions in the binary format
+
 - Error codes have been updated to be more accurate for the following error cases:
   - `getblock` now returns RPC_MISC_ERROR if the block can't be found on disk (for
   example if the block has been pruned). Previously returned RPC_INTERNAL_ERROR.