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/policy/policy.h

@@ -40,6 +40,7 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
                                                          SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS |
                                                          SCRIPT_VERIFY_CLEANSTACK |
                                                          SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY |
+                                                         SCRIPT_VERIFY_CHECKSEQUENCEVERIFY |
                                                          SCRIPT_VERIFY_LOW_S;
 
 /** For convenience, standard but not mandatory verify flags. */