Treat non-version-1 transactions as non-standard

Adds CBlock::CURRENT_VERSION and CTransaction::CURRENT_VERSION
constants, and makes non-CURRENT_VERSION transactions nonstandard.
This will help make future upgrades smoother.

src/main.cpp

@@ -268,6 +268,9 @@ bool CTransaction::ReadFromDisk(COutPoint prevout)
 
 bool CTransaction::IsStandard() const
 {
+    if (nVersion > CTransaction::CURRENT_VERSION)
+        return false;
+
     BOOST_FOREACH(const CTxIn& txin, vin)
     {
         // Biggest 'standard' txin is a 3-signature 3-of-3 CHECKMULTISIG