feat(feature-activation): new MUST_SIGNAL and LOCKED_IN states

[glevco]

Implementation of the design in HathorNetwork/rfcs#58

Acceptance Criteria

• Implement new MUST_SIGNAL and LOCKED_IN states
• Rename feature criteria's attribute from activate_on_timeout to lock_in_on_timeout
• Remove the minimum_activation_height rule that it couldn't be greater than the timeout_height
• Update timeout_height rule so it must be at least two evaluation intervals after start_height
• Update existing tests and add new ones

[codecov]

Codecov Report

Merging #675 (a515fb6) into master (e486924) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head a515fb6 differs from pull request most recent head a91c88c. Consider uploading reports for the commit a91c88c to get more accurate results

@@            Coverage Diff             @@
##           master     #675      +/-   ##
==========================================
- Coverage   84.13%   84.12%   -0.01%     
==========================================
  Files         246      246              
  Lines       20442    20446       +4     
  Branches     2757     2759       +2     
==========================================
+ Hits        17198    17201       +3     
- Misses       2672     2673       +1     
  Partials      572      572              

Impacted Files	Coverage Δ
hathor/feature_activation/feature_service.py	97.46% <100.00%> (+0.24%)	⬆️
hathor/feature_activation/model/criteria.py	100.00% <100.00%> (ø)
hathor/feature_activation/model/feature_state.py	100.00% <100.00%> (ø)
hathor/feature_activation/resources/feature.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes