Policy: allow transaction version 2 relay policy.

This commit introduces a way to gracefully bump the default
transaction version in a two step process.

src/policy/policy.cpp

@@ -55,7 +55,7 @@ bool IsStandard(const CScript& scriptPubKey, txnouttype& whichType)
 
 bool IsStandardTx(const CTransaction& tx, std::string& reason)
 {
-    if (tx.nVersion > CTransaction::CURRENT_VERSION || tx.nVersion < 1) {
+    if (tx.nVersion > CTransaction::MAX_STANDARD_VERSION || tx.nVersion < 1) {
         reason = "version";
         return false;
     }

src/primitives/transaction.h

@@ -206,8 +206,15 @@ private:
     void UpdateHash() const;
 
 public:
+    // Default transaction version.
     static const int32_t CURRENT_VERSION=1;
 
+    // Changing the default transaction version requires a two step process: first
+    // adapting relay policy by bumping MAX_STANDARD_VERSION, and then later date
+    // bumping the default CURRENT_VERSION at which point both CURRENT_VERSION and
+    // MAX_STANDARD_VERSION will be equal.
+    static const int32_t MAX_STANDARD_VERSION=2;
+
     // The local variables are made const to prevent unintended modification
     // without updating the cached hash value. However, CTransaction is not
     // actually immutable; deserialization and assignment are implemented,