More efficient header verification of headers for Parlia when snapshots are used (#3998)

* Update stageloop.go

* Print

* Consider snapshot headers as parlia checkpoints

* Not fail after not loading snapshot

* Lazy snapshots

* Print number of validators

* More printing

* Use epoch instead of checkpoint interval

* Reduce logging

* Fix compilation

* Remove trace jump dest

* Fix lint

* Not store snapshots every epoch

* Separate snapshot for verification and finalisation

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