feat(merged-mining): configure new max merkle path length on testnet

[glevco]

Depends on #931

Motivation

Create a new feature to change the maximum merkle path length of merged mining through Feature Activation, and configure it on testnet to start the process in roughly two weeks.

Acceptance Criteria

• Create INCREASE_MAX_MERKLE_PATH_LENGTH feature and configure it on testnet.
• Change BitcoinAuxPow.verify to receive a max_merkle_path_length instead of using a constant.
• Change MergeMinedBlockVerifier.verify_aux_pow to use Feature Activation to determine the max_merkle_path_length.
• Update tests.

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: 12 lines in your changes are missing coverage. Please review.

Comparison is base (b6748ee) 85.37% compared to head (a9a643d) 85.30%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
...n/storage/migrations/remove_second_nop_features.py	52.17%	11 Missing ⚠️
hathor/merged_mining/coordinator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #910      +/-   ##
==========================================
- Coverage   85.37%   85.30%   -0.07%     
==========================================
  Files         289      290       +1     
  Lines       22416    22454      +38     
  Branches     3374     3377       +3     
==========================================
+ Hits        19138    19155      +17     
- Misses       2608     2627      +19     
- Partials      670      672       +2     

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The base branch was changed.

The base branch was changed.