test: remove txid caching in CTransaction class

Rather than txids (represented by the fields `.sha256` and `.hash`)
being stateful, simply compute them on-the-fly. This ensures that
the correct values are always returned and takes the burden of
rehashing from test writers, making the code shorter overall.
In a first step, the fields are kept at the same name with @property
functions as drop-in replacements, for a minimal diff. In later commits,
the names are changed to be more descriptive and indicating the return
type of the txid.

test/functional/feature_block.py

@@ -1053,10 +1053,9 @@ class FullBlockTest(BitcoinTestFramework):
         self.log.info("Reject a block containing a transaction spending from a non-existent input")
         self.move_tip(69)
         self.next_block(70, spend=out[21])
-        bogus_tx = CTransaction()
-        bogus_tx.sha256 = uint256_from_str(b"23c70ed7c0506e9178fc1a987f40a33946d4ad4c962b5ae3a52546da53af0c5c")
+        bogus_txid_int = uint256_from_str(b"23c70ed7c0506e9178fc1a987f40a33946d4ad4c962b5ae3a52546da53af0c5c")
         tx = CTransaction()
-        tx.vin.append(CTxIn(COutPoint(bogus_tx.sha256, 0), b"", SEQUENCE_FINAL))
+        tx.vin.append(CTxIn(COutPoint(bogus_txid_int, 0), b"", SEQUENCE_FINAL))
         tx.vout.append(CTxOut(1, b""))
         b70 = self.update_block(70, [tx])
         self.send_blocks([b70], success=False, reject_reason='bad-txns-inputs-missingorspent', reconnect=True)