BIP112: Implement CHECKSEQUENCEVERIFY

- Replace NOP3 with CHECKSEQUENCEVERIFY (BIP112)
  <nSequence> CHECKSEQUENCEVERIFY -> <nSequence>
- Fails if txin.nSequence < nSequence, allowing funds of a txout to be locked for a number of blocks or a duration of time after its inclusion in a block.
- Pull most of CheckLockTime() out into VerifyLockTime(), a local function that will be reused for CheckSequence()
- Add bitwise AND operator to CScriptNum
- Enable CHECKSEQUENCEVERIFY as a standard script verify flag
- Transactions that fail CSV verification will be rejected from the mempool, making it easy to test the feature. However blocks containing "invalid" CSV-using transactions will still be accepted; this is *not* the soft-fork required to actually enable CSV for production use.

src/script/interpreter.h

@@ -81,6 +81,11 @@ enum
     //
     // See BIP65 for details.
     SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY = (1U << 9),
+
+    // support CHECKSEQUENCEVERIFY opcode
+    //
+    // See BIP112 for details
+    SCRIPT_VERIFY_CHECKSEQUENCEVERIFY = (1U << 10),
 };
 
 bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned int flags, ScriptError* serror);
@@ -100,6 +105,11 @@ public:
          return false;
     }
 
+    virtual bool CheckSequence(const CScriptNum& nSequence) const
+    {
+         return false;
+    }
+
     virtual ~BaseSignatureChecker() {}
 };
 
@@ -116,6 +126,7 @@ public:
     TransactionSignatureChecker(const CTransaction* txToIn, unsigned int nInIn) : txTo(txToIn), nIn(nInIn) {}
     bool CheckSig(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode) const;
     bool CheckLockTime(const CScriptNum& nLockTime) const;
+    bool CheckSequence(const CScriptNum& nSequence) const;
 };
 
 class MutableTransactionSignatureChecker : public TransactionSignatureChecker