feat(consensus): eliminate block ties

[msbrogli]

Motivation

We've always supported "block ties", meaning that blocks that have the same score will tie and both void each other, while at the same time having to remain as "best block tips" (which always had to be a list). This pushes some unnecessary complexity throughout the code. There are no disadvantages to using the block hash as a tie-breaker and simplifying this complexity.

In practice there's a slight difference in the consensus of certain situations, the difference is that before some tied block chains would be voided all the way back until they weren't tied anymore, now one of them would be picked as best chain. This situation does not cause any incompatibility in how nodes sync and any new block with higher score will cause the same state on both chains.

This PR is the first step, that only focuses on removing ties at the consensus level. Subsequent PRs will focus on refactoring out the complexity of supporting multiple tips.

Acceptance criteria

• update block consensus' update_voided_info so when testing if a new block might be a winner, and its score is neither higher or lower than the current winner (that is, a tie), the hash is used as a tie breaker
• update block consensus' update_voided_info so when a new block is not a winner, only the tip with highest score and lowest hash is stored as "best block tips" cache
• update transaction storage's get_best_block_tips so it only ever returns a list of size 1, when the block tips index returns more than one tip with the highest score, the one if lowest hash is selected
• update test_split_brain_only_blocks_different_height so it checks the node will correctly not void everything back to the genesis and tie the two blocks (when before it used to)
• update test_single_fork_not_best similarly, so it checks that the best chain is considered

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.19%. Comparing base (623856d) to head (5e3d848).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
hathor/transaction/storage/transaction_storage.py	0.00%	1 Missing and 1 partial ⚠️
hathor/consensus/block_consensus.py	97.22%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
- Coverage   86.23%   86.19%   -0.05%     
==========================================
  Files         440      440              
  Lines       34068    34056      -12     
  Branches     5330     5327       -3     
==========================================
- Hits        29378    29353      -25     
- Misses       3668     3677       +9     
- Partials     1022     1026       +4     

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[msbrogli]

Should we create an issue to refactor this method and simply store the current best block in the storage?

[jansegre]

#540 sort of does this by using the height-index, but because it's not using this PR as a base it does this incorrectly and hides other tips, after this PR that change would be correct

[jansegre]

#540 was superseded by #1200 which just removes sync-v1 indexes instead of making them optional, because sync-v1 has already been removed.

[jansegre]

#1200 uses the block-height index to get the best block now, which will now always match with the best-block-tip since they can't be voided because of a tie

[msbrogli]

This test needs to be fixed.

[msbrogli]

This test needs to be fixed.

[jansegre]

We still need to have the tests passing, but I'm OK with the implementation.