feat(sync): bifurcation for syncTarget

[cristaloleg]

Fixes #217

[renaynay]

more comments for tests would be helpful - and I think there's some deduplication that can be done throughout the tests if possible ideally but that's just a nice to have.

TODO:

• some sort of metric reported if verifySkipping fails
• reject networkHead in case verifySkipping fails (log it aggressively w/ a WARN and report as metric)
• change name of err message

[renaynay]

Very nice coverage with tests. 2 remaining comments to address

[Wondertan]

Finally, I get to this. Apologies for taking it that long.

The logic is correct, and test coverage is great. I have a few semantical comments on error handling, comments and metrics.

I also had an idea on how to make bifurcation part of a general code path with recursion, but that would require minor refactoring of existing methods. It could make the code a bit cleaner, but the impact of spending more time on this is really low, so lets keep it as it is.

[Wondertan]

One thing I recalled. Only mocha currently has this case, and we should test the patch there before merging by detecting if bifurcation happened and succeeded.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 63.63636% with 24 lines in your changes missing coverage. Please review.

Project coverage is 64.01%. Comparing base (88c5b8c) to head (7eec051).
Report is 25 commits behind head on main.

Files with missing lines	Patch %	Lines
sync/metrics.go	5.26%	17 Missing and 1 partial ⚠️
sync/sync_head.go	86.36%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
+ Coverage   62.80%   64.01%   +1.21%     
==========================================
  Files          39       38       -1     
  Lines        3589     3702     +113     
==========================================
+ Hits         2254     2370     +116     
  Misses       1160     1160              
+ Partials      175      172       -3     

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