Fix next header hash may be misset on resume.

[asdacap]

• Seems like it does not reinitialize to the right header. This might cause the sync to hang as it failed validation, which also trigger a disconnect. Seen on sepolia, although it tend to recover on its own eventually.
• Previously, this would only get triggered on restart.

Changes

• Make sure on restart the fast header sync set the next expected header to parent of the lowest inserted header instead of the lowest inserted header.

Types of changes

What types of changes does your code introduce?

• Bugfix (a non-breaking change that fixes an issue)

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Notes on testing

• Tested on sepolia