SEC-13 Parent issue for all uncle validation / logic security issues

[Gustav-Simonsson]

• Fix is-kin property: SEC-11 Uncle validation does not correctly implement is-kin property #387
• Ensure uncle header validation: SEC-10 Uncle validation does not include all parts of block header validity function #381
• Ensure uncle uniqueness (a block cannot be uncle more than one time)

[Gustav-Simonsson]

Re-opening since we still do not have working block (chain) tests for this.