Terminal block hash validation

[asdacap]

Validate terminal blockhash if explicitly specified.

Changes:

• Added a validation in HeaderValidator to check if terminal block have the same hash as configured.
• Can't really set a global flag because then it can't suggest a different FcU head to change to a different branch meaning we can't verify it if we are syncing far behind because invalid chain tracker have limited tracking.

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation Update
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Other (please describe):