Engine API: replace INVALID_TERMINAL_BLOCK with INVALID + lvh=0x00..00

[mkalinin]

Implements Option 3 from #212, that is:

• Replace INVALID_TERMINAL_BLOCK status with {status: INVALID, latestValidHash=0x00..00} which simplifies the spec and CL implementations
• Fix a blind spot in the spec when merge transition block is itself INVALID by returning {status: INVALID, latestValidHash=0x00..00}

A list of places where according to this change EL must respond {status: INVALID, latestValidHash=0x00..00}:

• Replacement of INVALID_TERMINAL_BLOCK
  • Invalid terminal PoW block is encountered during payload validation process
  • Invalid terminal PoW is referenced by headBlockHash in fcU call
• New requirement
  • Invalid merge transition block is encountered during payload validation process

This proposal does not break existing testnets which have already transitioned to PoS e.g. Kiln.

Note: INVALID_TERMINAL_BLOCK status should be removed from Optimistic Sync spec if this PR is merged.

[djrtwo]

I'm okay with this