add file and db backend for dump-block feature

[bilboquet]

• [ ] document all added functions
• [ ] try in sandbox /simulation/labnet
  • [ ] if part of node-launch, checked using the resync_check flag
• [ ] unit tests on the added/changed features
  • [ ] make tests compile
  • [ ] make tests pass
• [ ] add logs allowing easy debugging in case the changes caused problems
• [ ] if the API has changed, update the API specification