Add option to activate hardfork using UNIX timestamp

[Wodann]

In EDR, we currently support configuration of hardfork activations using block numbers only. More commonly, UNIX timestamp are used to determine when blocks need to start being mined with a new hardfork.

Definition of Done

• Add the ability to activate hardforks using a UNIX timestamp
• Hardfork activation configuration should be mutually exclusive: either by timestamp or by block number
• Update hardfork activations to use timestamps, where applicable.

[alcuadrado]

Update hardfork activations to use timestamps, where applicable.

Every hardfork before the merge uses block numbers. Every hardfork after it uses timestamps. I don't remember what was the criteria for the merge itself.

[fvictorio]

The merge used difficulty, as far as I remember. I guess that's an edge case we can ignore.