SAN DIEGO — Considering that his father’s band performed immediately before Jimi Hendrix took the stage at the legendary Woodstock festival in 1969, it’s not surprising Brian Witkin grew up dreaming of becoming a rock star. What is surprising is that this 38-year-old law school graduate — the founder of San Diego’s 20-year-old Pacific Records label — is commanding the spotlight by playing traditional Hawaiian music. And that he’s doing so leading an award-winning band, Slack Key Ohana, whose lineup features his retired emergency-room doctor dad Joe on bass ukulele and his retired travel agent,…