ChainSegment efficiency (#3042)

* 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