Fix datarace while update a channel.lastEventID

Protect the option by wrappring into a mutex. The datarace is gone.
It closes alexandrevicenzi#18

channel.go

@@ -23,7 +23,9 @@ func newChannel(name string) *Channel {
 
 // SendMessage broadcast a message to all clients in a channel.
 func (c *Channel) SendMessage(message *Message) {
+	c.mu.Lock()
 	c.lastEventID = message.id
+	c.mu.Unlock()
 
 	c.mu.RLock()
 

sse_test.go

@@ -90,8 +90,9 @@ func TestServer(t *testing.T) {
 }
 
 func TestMultipleTopics(t *testing.T) {
-	// usage pattern we have a client which subsribed to multiple channels
-	// in one connection
+	// The usage pattern: Imagine  we have a client which subsribed to multiple topics
+	// inside a connection. It track changes of specific items state by their ID
+	// for example.
 	sendersWg := sync.WaitGroup{}
 	workerWg := sync.WaitGroup{}
 	m := sync.Mutex{}