Use a database entry to hold first non origin slot number after finalization

[EclesioMeloJunior]

Description

• This is needed for full nodes that don't keep block history to hold the very first non origin block slot number in a case where block #1 is pruned from disk.

• What should be done here is:

  • When finalization happens you should check if one of the blocks being finalized is block #1 if so you should persist its slot number in the database, then everytime you want to get the chain first slot you should use that value
  • PR feat(babe, epoch): retrieve first slot number correctly #3841 already addresses the case where finalization did not happen and there is no chain first slot persisted on DB.
  • On the node initialization you should check if the entry is in the database, if so you should retrieve it and cache it, you can place the persistency under dot/state/slot.go as well as field to hold the persisted chain first slot.

• Depends on fix(lib/epoch): use correctly very first slot number #3795

• More context about epoch changes: https://hackmd.io/@-8f0KkDaSXGl-AgsJkF8Jg/B1LIdTxAa

• This task should be done on top of branch eclesio/move-slot-to-header or after the merge of PR feat(babe, epoch): retrieve first slot number correctly #3841

• Maybe check in the retrieveFirstNonOriginBlockSlot if the entry is in the database