WIP: make sure validate_header also validates seal on parent

[cburgdorf]

What was wrong?

In CliqueConsensus we can not validate a seal on a header if we haven't validated the seal of the parent. But that's exactly what can happen when using validate_header. This was causing a bug during Görli sync that I just had ignored so far.

How was it fixed?

THE IMPLEMENTATION IS JUST AN EXAMPLE

We probably want to fix that on the Clique level to not waste cpu cycles when it's not needed.

I'm going to wait until the dust around #1874 / #1875 has settled before I attempt to send a proper fix.

To-Do

• [ ] Clean up commit history

• [ ] Add entry to the release notes

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