Extract yorkieServer and clusterServer from rpc.Server (yorkie-team#200)

Co-authored-by: Hackerwins <[email protected]>

yorkie/rpc/cluster.go → yorkie/rpc/cluster_server.go

@@ -7,10 +7,21 @@ import (
 	"github.com/yorkie-team/yorkie/api/converter"
 	"github.com/yorkie-team/yorkie/internal/log"
 	"github.com/yorkie-team/yorkie/pkg/document/time"
+	"github.com/yorkie-team/yorkie/yorkie/backend"
 )
 
+// clusterServer is a normal server that processes the broadcast by the agent.
+type clusterServer struct {
+	backend *backend.Backend
+}
+
+// newClusterServer creates a new instance of clusterServer.
+func newClusterServer(be *backend.Backend) *clusterServer {
+	return &clusterServer{backend: be}
+}
+
 // BroadcastEvent publishes the given event to the given Topic.
-func (s *Server) BroadcastEvent(
+func (s *clusterServer) BroadcastEvent(
 	ctx context.Context,
 	request *api.BroadcastEventRequest,
 ) (*api.BroadcastEventResponse, error) {