Change/use persisted pointer for fast header

[asdacap]

• Currently unlike oldbodies, orldreceipt or beacon header, the pointer for fast header is initialized via binary search from genesis to sync pivot.
• This cause a problem with era/portal which may insert header in between arbitrarily.
• This change the pointer to be persisted like beacon header and the pointer is changed manually in fast header instead of on every block/header insert.
• Also don't download already persisted header. This only 2x faster than downloading though.

Types of changes

What types of changes does your code introduce?

• Optimization
• Refactoring

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

• Mainnet sync work.