This commit updates the timestamp updating logic
such that `time.Now` is used instead of relying on
L1 timestamps. This gives a higher fidelity for the
`TIMESTAMP` opcode as well as makes the time on L2
be closer to the time on L1.

L1 to L2 transactions no longer have the property of
having the same timestamp on L2 as the timestamp
of the L1 block they were included in.

This should be able to be turned on without needing
hardfork logic as replicas should always accept the
timestamp that the sequencer sets. The sequencer is
a trusted entity in the existing implementation and
it is expected that the sequencer will become more
trustless in future iterations of the protocol.

This change is added to improve both UX and devex.
Users are confused by the timestamps on Etherscan
being ~15 minutes behind. This is due to the timestamps
being set based on L1 block numbers, and the system
only pulls L1 data once a secure amount of PoW
is placed on top. Developers would like the timestamps
to have a higher fidelity and be closer to the timestamps
on L1.