Add cluster replication features and membership management

- Introduced a new `ClusterManager` to handle cluster membership events and facilitate event replication across relay peers.
- Implemented HTTP endpoints for retrieving the latest serial and fetching events within a specified range.
- Enhanced event handling to process cluster membership events (Kind 39108) and update relay lists accordingly.
- Updated configuration to support cluster administrators and their management capabilities.
- Added comprehensive tests to validate the new cluster replication functionalities.
- Documented the cluster replication protocol in a new specification file.
- Bumped version to reflect these changes.

app/server.go

@@ -53,6 +53,7 @@ type Server struct {
 	spiderManager    *spider.Spider
 	syncManager      *dsync.Manager
 	relayGroupMgr    *dsync.RelayGroupManager
+	clusterManager   *dsync.ClusterManager
 	blossomServer    *blossom.Server
 }
 
@@ -259,6 +260,13 @@ func (s *Server) UserInterface() {
 		s.mux.HandleFunc("/blossom/", s.blossomHandler)
 		log.Printf("Blossom blob storage API enabled at /blossom")
 	}
+
+	// Cluster replication API endpoints
+	if s.clusterManager != nil {
+		s.mux.HandleFunc("/cluster/latest", s.clusterManager.HandleLatestSerial)
+		s.mux.HandleFunc("/cluster/events", s.clusterManager.HandleEventsRange)
+		log.Printf("Cluster replication API enabled at /cluster")
+	}
 }
 
 // handleFavicon serves orly-favicon.png as favicon.ico