Increment penalty in case of repeated violation during previous duration

[vzotova]

Type of PR:

• Bugfix
• Feature
• Documentation
• Other

Required reviews:

• 1
• 2
• 3

What this does:
See #259

Issues fixed/closed:
Fixes #259

Why it's needed:

Explain how this PR fits in the greater context of the NuCypher Network.
E.g., if this PR address a nucypher/productdev issue, let reviewers know!

Notes for reviewers:

What should reviewers focus on?
Is there a particular commit/function/section of your PR that requires more attention from reviewers?

[manumonti]

LGTM! 👌

[derekpierre]

🎸

[arjunhassard]

Nice work! One future-proofing comment; at some point it would be useful to add a variable – violationTimeframe or something, which can be tuned so that if there are lots of DKG incompletions within a short timeframe (e.g. 3 hours), the guilty staker is given a reprieve from penalty increments, and only sees their rewards reduced by the default (unless they were already within a penalty period, of course!).