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.