Make LeaderElection.runElection async

[Konrad `ktoso` Malawski]

public protocol LeaderElection {
    /// Select a member to become a leader out of the existing `Membership`.
    ///
    /// Decisions about electing/selecting a leader may be performed asynchronously.
    mutating func runElection(context: LeaderElectionContext, membership: Cluster.Membership) -> LeaderElectionResult
}

should be async