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.