Decouple GAS Reward from block index to block timestamp

[vncoelho]

Summary or problem description
We should decouple the GAS reward to be calculated by block index to block timestamp.

#4280 (comment)

Do you have any solution you want to propose?
There is the challenge of the speakers proposing timestamps, on the other hand, we currently already have mechanisms that limits that acceptance.

So, the steps to achieve that may be:
[ ] fine-tuning the timestamp limits proposed by the speakers
[ ] discuss the creation of GAS rewards limits in order to protect this GAS rewards generation and motivate fair timestamps

Where in the software does this update applies to?

• Consensus
• Ledger

[shargon]

More than timestamp (that is controlled) depend the expected block time