* added download channel to stage headers

* added direct execution

* now channel of headers rather than bodies

* simplifications

* fixed ttd headerForward

* garbage removed

* added some more stepping stones

* better transition

* covered edge case

* added names to returned values

* clean

Bump EthBackend API to 3.0.0 in `devel` branch only to make it explicitly incompatible with `stable` (#3077)

* fixed SpawnStageHeader not passing the test

* gofmt

* Update Engine API to v1.0.0-alpha.5

* Remove ForkchoiceState since we're going to handle unwinds via ExecutePayload, not ForkchoiceUpdated

* Remove deduplication in NodeInfo response

* remove unused dependencies

* [EIP-4399] Remove newly added Header.Random (reuse mixHash instead)

* add a TODO

* Update mock payload hashes

Co-authored-by: Alex Sharp <alexsharp@Alexs-MacBook-Pro.local>

* Change eth.Peer.ID() type from string to enode.ID

* Misc enode.ID.String() usage

* Add NodeInfo implementation

* replace magic value with a constant.

* update dependencies

* bump minor version

* add nodes deduplication logic.

* shuffle values in test cases a little.

Added ExecutePayloadV1 to engine_ namespace [the merge]

* Comment out comparisons

* Introduce check option

Co-authored-by: Alexey Sharp <alexeysharp@Alexeys-iMac.local>
Co-authored-by: Alex Sharp <alexsharp@Alexs-MacBook-Pro.local>

* Fix Header.EncodingSize() to match EncodeRLP() with EIP-3675

* Fix error messages for Random in DecodeRLP()

Co-authored-by: Alex Sharov <AskAlexSharov@gmail.com>

* Prefer uint256.IsZero() or big.Sign() for 0-testing over BitLen()

* Avoid redundant BitLen()

* Create access list

* CreateAccessList compiles

* fixes

Co-authored-by: Alexey Sharp <alexeysharp@Alexeys-iMac.local>

* Add separate Block.Nonce() and Block.NonceU64()

* Add Block.Seal()

* Avoid redundant Block.Header() deep-copy

* Add warning comment for Block.Header()

Co-authored-by: Alexey Sharp <alexeysharp@Alexeys-iMac.local>

* Extract pruneLinkQueue() out of ProcessSegment()

* Use sub-slices for segment actions

* De-dup blockHeaders66() and blockHeaders65()

* Simplify loops and EOL detection

* Add ChainSegmentHeader struct and refactor

* Add RawRlpHash() to avoid re-encode for header hash

* Avoid multiple redundant rlpHash()

* Sort headers by height,hash to make dups consecutive

* Flip condition to reduce map lookups

* Remove redundant check

* Use rlp.RawValue rather than []byte to help self-document