Bound lookup parent chain length with tip extension

[dapplion]

Issue Addressed

Sync lookup bounds the maximum length of parent chains. Current code covered the case where a new parent lookup extended the ancestor of a parent chain beyond the max length. However, there's a second case. Consider these events:

• New lookup for unknown block root
• Receive block root which happens to be the child of a parent's chain tip
• Parent chain exceeds max length

Proposed Changes

Bound lookup parent chain length with tip extension. Extending the sequence of events above:

• New lookup for unknown block root
• Receive block root which happens to be the child of a parent's chain tip
• Send block for processing
• Receive ParentUnknown
• Call search_parent_of_child(parent, block)

This PR introduces the max length bound in this final step