DES MOINES — Regardless of what happens in this fall’s elections, Democrats in the Iowa Senate are about to lose a mountain of institutional knowledge: None of the three longest-serving Democrats there will be back next year.Wally Horn and Bob Dvorsky have announced their retirement, and Matt McCoy is not running for re-election so he can instead run for the Polk County Board of Supervisors.Horn is the longest continually serving state legislator in Iowa history.