fix(feature-activation): add missing metadata migration

[glevco]

Motivation

During the latest release, we noticed that full node initialization could fail depending on the snapshot state (only on testnet). This was caused by a missing migration on the new Feature Activation metadata attribute.

Acceptance Criteria

• Rename BitSignalingService.start() to generate_initial_logs() and add docstring. Also move call to after migrations are applied in HathorManager.
• Implement add_feature_activation_bit_counts_metadata migration.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (ab1a6ca) 84.63% compared to head (559d250) 84.62%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #828      +/-   ##
==========================================
- Coverage   84.63%   84.62%   -0.01%     
==========================================
  Files         270      271       +1     
  Lines       22290    22306      +16     
  Branches     3404     3406       +2     
==========================================
+ Hits        18865    18877      +12     
- Misses       2757     2762       +5     
+ Partials      668      667       -1     

Files	Coverage Δ
hathor/feature_activation/bit_signaling_service.py	85.24% <ø> (ø)
hathor/manager.py	80.73% <100.00%> (ø)
hathor/transaction/storage/transaction_storage.py	93.17% <100.00%> (+0.12%)	⬆️
...ions/add_feature_activation_bit_counts_metadata.py	68.75% <68.75%> (ø)

... and 2 files with indirect coverage changes

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