setLockTime

Optional lockTime, default value 0 means no time locking.
If all inputs have final (`0xffffffff`) sequence numbers then `lockTime` is irrelevant.
Otherwise, the transaction may not be added to a block until after `lockTime`.
 value  < 500000000 : Block number at which this transaction is unlocked
 value >= 500000000 : UNIX timestamp at which this transaction is unlocked