Disallow v0 transactions as a consensus rule

Closes #1557

src/main.cpp

@@ -851,6 +851,12 @@ bool CheckTransactionWithoutProofVerification(const CTransaction& tx, CValidatio
 {
     // Basic checks that don't depend on any context
 
+    // Check transaction version
+    if (tx.nVersion < MIN_TX_VERSION) {
+        return state.DoS(100, error("CheckTransaction(): version too low"),
+                         REJECT_INVALID, "bad-version-too-low");
+    }
+
     // Transactions can contain empty `vin` and `vout` so long as
     // `vjoinsplit` is non-empty.
     if (tx.vin.empty() && tx.vjoinsplit.empty())