[Leadership] Deprecate leadership protocol, call it Legacy

Author: ktoso

Sources/DistributedCluster/Cluster/Leadership.swift

@@ -42,11 +42,12 @@ import NIO // Future
 /// If a new member is selected as leader, a ``Cluster/Event`` carrying ``Cluster/LeadershipChange`` will be emitted.
 /// Other actors may subscribe to `ClusterSystem.cluster.events` in order to receive and react to such changes,
 /// e.g. if an actor should only perform its duties if it is residing on the current leader node.
-public protocol LeaderElection {
+public protocol LegacyLeaderElection {
     /// 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
+
 }
 
 public struct LeaderElectionContext {
@@ -103,9 +104,9 @@ extension Leadership {
         static let naming: _ActorNaming = "leadership"
 
         private var membership: Cluster.Membership // FIXME: we need to ensure the membership is always up to date -- we need the initial snapshot or a diff from a zero state etc.
-        private var election: LeaderElection
+        private var election: LegacyLeaderElection
 
-        init(_ election: LeaderElection) {
+        init(_ election: LegacyLeaderElection) {
             self.election = election
             self.membership = .empty
         }
@@ -227,7 +228,7 @@ extension Leadership {
     /// fulfilling this role whenever the minimum number of nodes exist. This may be useful when operation would
     /// potentially be unsafe given less than `minimumNrOfMembers` nodes.
     ///
-    public struct LowestReachableMember: LeaderElection {
+    public struct LowestReachableMember: LegacyLeaderElection {
         // TODO: In situations which need strong guarantees, this leadership election scheme does NOT provide strong enough
         // guarantees, and you should consider using another scheme or consensus based modes.
         let minimumNumberOfMembersToDecide: Int
@@ -331,7 +332,7 @@ extension ClusterSystemSettings {
 
         private let underlying: _LeadershipSelectionSettings
 
-        func make(_: ClusterSystemSettings) -> LeaderElection? {
+        func make(_: ClusterSystemSettings) -> LegacyLeaderElection? {
             switch self.underlying {
             case .none:
                 return nil