Blacklist improvements

[nritsche]

Problem:
Nodes get blacklisted for various reasons and we need some way to attach the reason to the changes of the blacklist so that it's easier to follow up on it.

Solutions:

• Add command: blacklist for a limited time
• Add save/load blacklist functionality
  • Very easy to implement
  • The issue with that is that you don't know how far back to undo, e.g. imagine that this happens:
    • Black list a rack because it's bad
    • Black list a node because it's bad
    • Black list another rack because it's bad
    • Niko fixes both bad racks
    • Now which state do you revert back to?
• Multiple levels of blacklists (hierarchy)
• Multiple named blacklists