Explicitly pass parent header in header validator

[asdacap]

• Explicitly pass parent header in header validator.
  • Otherwise previously it will use BlockHeader.MaybeParent, which is just too implicit or _blockTree.FindHeader.
• Effect block validator as well.

Types of changes

What types of changes does your code introduce?

• Refactoring

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

• Mainnet can sync normally.
• Mainnet era replay runs fine.