Move stepdowns of streams and consumers to not be inline. (#4029)

Signed-off-by: Derek Collison <derek@nats.io>

server/jetstream_api.go

@@ -1967,18 +1967,24 @@ func (s *Server) jsStreamLeaderStepDownRequest(sub *subscription, c *client, _ *
 		return
 	}
 
-	// Call actual stepdown.
-	if mset != nil {
+	if mset == nil {
+		resp.Success = true
+		s.sendAPIResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(resp))
+		return
+	}
+
+	// Call actual stepdown. Do this in a Go routine.
+	go func() {
 		if node := mset.raftNode(); node != nil {
 			mset.setLeader(false)
 			// TODO (mh) eventually make sure all go routines exited and all channels are cleared
 			time.Sleep(250 * time.Millisecond)
 			node.StepDown()
 		}
-	}
 
-	resp.Success = true
-	s.sendAPIResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(resp))
+		resp.Success = true
+		s.sendAPIResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(resp))
+	}()
 }
 
 // Request to have a consumer leader stepdown.
@@ -2073,16 +2079,23 @@ func (s *Server) jsConsumerLeaderStepDownRequest(sub *subscription, c *client, _
 		return
 	}
 
-	// Call actual stepdown.
-	if n := o.raftNode(); n != nil {
+	n := o.raftNode()
+	if n == nil {
+		resp.Success = true
+		s.sendAPIResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(resp))
+		return
+	}
+
+	// Call actual stepdown. Do this in a Go routine.
+	go func() {
 		o.setLeader(false)
 		// TODO (mh) eventually make sure all go routines exited and all channels are cleared
 		time.Sleep(250 * time.Millisecond)
 		n.StepDown()
-	}
 
-	resp.Success = true
-	s.sendAPIResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(resp))
+		resp.Success = true
+		s.sendAPIResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(resp))
+	}()
 }
 
 // Request to remove a peer from a clustered stream.