Replace NOP2 with CHECKLOCKTIMEVERIFY

<nLockTime> CHECKLOCKTIMEVERIFY -> <nLockTime>

Fails if tx.nLockTime < nLockTime, allowing the funds in a txout to be
locked until some block height or block time in the future is reached.

Only the logic and unittests are implemented; this commit does not have
any actual soft-fork logic in it.

Credit goes to Gregory Maxwell for the suggestion of comparing the
argument against the transaction nLockTime rather than the current
time/blockheight directly.

Credit goes to Pieter Wuille for suggesting integration with
CheckSignature().
Enable block nVersion=3 CHECKLOCKTIMEVERIFY upgrade process