Verify PoS headers before inserting them into DB (#3151)

* Remove TotalDifficulty stage

* Introduce FeedHeaderFuncType

* Some TODOs

* Don't gossip PoS headers

* FeedHeaderPoS

* Call header.Hash once

* Use FeedHeaderPoS for to write PoS header

* Support errors in EngineExecutePayloadV1

* useExternalTx boilerplate

* Call VerifyHeader on PoS header

* latestValidHash must be nil when status is SYNCING

* fixCanonicalChain instead of canonicalHeadersCollector

* engine.VerifyHeader + FeedHeaderPoS as headerLoadFunc

* fix daemon

Co-authored-by: Giulio Rebuffo <giulio.rebuffo@gmail.com>