New Engine API semantics (Kiln v1) (#3340)

* Disable PoS sync temporarily

* Resore PoS sync

* Handle Ctrl^C in HeadersPOS

* Consistent naming

* Extract verifyAndSavePoSHeader & downloadMissingPoSHeaders

* Preparation for EngineForkChoiceUpdated re-orgs

* Extract ForkingPoint

* Comments

* New proto for Engine API

* EngineExecutePayload -> EngineNewPayload

* Return INVALID_BLOCK_HASH if block hash is invalid

* Return EngineStatus_ACCEPTED for side chain blocks

* Update erigon-lib (PR 268)

* Fix payload2Hash

* reverseDownloadCh -> beaconPayloadCh

* Update erigon-lib

* Engine API updated

* ExecutionStatus -> PayloadStatus

* Introduce forkChoiceCh

* Mock ForkChoiceMessage/PayloadStatus

* Add ValidationError to PayloadStatus

* Small clean-ups

* Add INVALID_TERMINAL_BLOCK to EngineStatus

* Add a comment

* Extract handleNewPayload & handleForkChoice

* Partially implement handleForkChoice

* Update erigon-lib

* short vs long re-org

* Move header insertion out of downloadMissingPoSHeaders

* Update erigon-lib

* Refactor ProcessSegmentPOS

* Fix imports

* Wire downloadMissingPoSHeaders into handleForkChoice

* evictOldPendingPayloads

* nolint:unused for assertSegment

* Try nolint instead of nolint:unused

* Small comment improvements

* HeadHeaderHash/StageProgress in handleForkChoice

* TODO: bodyDownloader.AddToPrefetch(block)

* Review call suggestions

* Don't use ReadHeaderNumber in ProcessSegmentPOS

* Don't leave ethbackend waiting when server is stopping

* Update erigon-lib

* More explicit signature of downloadMissingPoSHeaders