Override the CURRENT_TIMESTAMP and store in checkpoint

[kennytm]

After #170, Lightning can recognize the DEFAULT CURRENT_TIMESTAMP option, which is always initialized to time.Now(). This means restoring from checkpoint is going to be non-deterministic when the timestamp column is indexed.

CREATE TABLE t (
    x DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
    KEY(x)
);
INSERT INTO t VALUES (), (), (), ();

We need to override the session system variable timestamp, and also store it into the checkpoint to ensure the value before and after are equivalent.