For more than four decades, the DNA sat on the top shelf of the Orange County Crime Lab – evidence the killer left behind on the dress of a Corona del Mar girl found strangled to death in a ditch in Newport Beach’s Back Bay on July 7, 1973. For years, the abduction and slaying of 11-year-old Linda Ann O’Keefe haunted the Newport Beach PD and community. Now, thanks to cutting-edge DNA analysis and genetic geneology techniques that were unimaginable back in the Nixon era, Orange County law enforcement officials have arrested a Colorado man they believe is responsible for…