Timestamp-based forking for Shanghai (and later)

[AdamSpitz]

Activation of the Shanghai fork will be determined by a timestamp, not a block number.

e.g. EIP-4895 says, "Beginning with the execution timestamp FORK_TIMESTAMP, execution clients MUST introduce the following extensions to payload validation and processing..."

jangko had some comments here on how to go about implementing this.

• We probably need both shanghaiBlock and shanghaiTime, because hive and ethereum/tests are still using a block number.
• The relevant files are common/common.nim and hardforks.nim.

[AdamSpitz]

Note also that geth has put in a hard-coded value called timestampThreshold, used to decide whether a forkid.next field is a block number or a timestamp.

https://github.com/ethereum/go-ethereum/blob/master/core/forkid/forkid.go#L48

[jangko]

in EIP-4895, geth still keep both shanghaiBlock and shanghaiTime. but in EIP-4844, as can be seen in this commit, they remove shanghaiBlock and use only shanghaiTime.
Sure this will complicated fork transition and forkid calculation.

[jangko]

fixed by #1465. closing