Fix syncing regressions from #841

[austinabell]

Summary of changes
Changes introduced in this pull request:

Regressions from #841:

• Version getter was broken (I removed the unnecessary parts added to it, since it was very clunky and I don't know how I missed in review)
  • It was checking when the epoch == the fork epoch, not < or whatever would have been required with that setup
• Incorrect tipsets were being passed back from getting lookback
• If statement was wrong logic
• getting the next tipset was not incrementing epoch by one

Mb for missing these in review and not doing a more thorough test, but this only gets caught when doing a full sync, and we can't have that test run by default on CI because of network calls necessary

Reference issue to close (if applicable)

Closes

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