[FEATURE] Change FreezeLog to a "pure" time lock specification

[YoshihitoAso]

Is your feature request related to a problem? Please describe.

The current FreezeLog contract specification control the update so that it cannot be updated once a specified number of blocks has arrived, rather than at a specific time.

However, counting up the number of blocks is sensitive to the operating status of the blockchain.
Therefore, I would like to lock purely by time rather than by block number.

Also, by locking by time, there will be no problem even if emptyBlockPeriod is introduced in the ibet network in the future.

Describe the solution you'd like

I'm thinking of using block.timestamp.
In the consortium chain, the block.timestamp setting value is trustworthy, so we believe that there is no problem in using it.