Hardcode terminal total difficulty

[mkalinin]

What's done?

• TransitionStore with compute_terminal_total_difficulty function are replaced with TERMINAL_TOTAL_DIFFICULTY configuration parameter
• Adjustments to different parts of the spec that are corresponding to the above change

Rationale

• Reduce the complexity of consensus client implementation caused by computed TTD, see Terminal Total Difficulty calculation may differ between nodes #2603 for details.
• Hardcoded TTD is a robust solution to prevent execution client from mistakenly following the contentious fork sharing the Mainnet PoW chain in the case of software misconfiguration or failure.

Dependencies

• The corresponding change should be made to EIP-3675 -- EIP-3675: Hardcode terminal total difficulty EIPs#3981
• Should be merged after Set the value of the random field to the previous randao_mix #2581

cc @ajsutton

[djrtwo]

a couple of small notes about config.

otherwise looks good!
will wait on #2581 to approve so the diff looks cleaner for final review

[djrtwo]

excellent!

[lsankar4033]

nice! this is definitely cleaner. re: #2599, do you think it makes sense to have a param for TERMINAL_BLOCK_HASH as well?

[mkalinin]

nice! this is definitely cleaner. re: #2599, do you think it makes sense to have a param for TERMINAL_BLOCK_HASH as well?

Yes, I think this would be the right way to implement override of a terminal block. And if transition ends up with terminal block override then consensus and execution clients will need to be released with hardcoded terminal block hash.