db: Add utitlities to ban vtxos

Erik De Smedtrequested to merge
erik/sec-22-banning-vtxos into master

Alternative to @2Fast2BCn work. The key design decisions are

  • do not use a local cache for banned vtxos. It is complex and adds little
  • include is_banned in the is_spendable check
  • banning occurs by block-height. This ensures we cannot ban after vtxo expiry by accident
  • check or remove this line after you've added a CHANGELOG entry or if your PR doesn't need one (they should go into CHANGELOG/unreleased/)

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