Minimize client lock duration (#223)

* Minimize client lock duration
* Fix server option example

README.md

@@ -136,13 +136,13 @@ A number of configurable options are available which can be used to alter the be
 ```go
 server := mqtt.New(&mqtt.Options{
   Capabilities: mqtt.Capabilities{
-    ClientNetWriteBufferSize: 4096,
-    ClientNetReadBufferSize: 4096,
     MaximumSessionExpiryInterval: 3600,
     Compatibilities: mqtt.Compatibilities{
       ObscureNotAuthorized: true,
     },
   },
+  ClientNetWriteBufferSize: 4096,
+  ClientNetReadBufferSize: 4096,
   SysTopicResendInterval: 10,
 })
 ```

clients.go

@@ -481,6 +481,14 @@ func (cl *Client) ReadPacket(fh *packets.FixedHeader) (pk packets.Packet, err er
 
 // WritePacket encodes and writes a packet to the client.
 func (cl *Client) WritePacket(pk packets.Packet) error {
+	if cl.Closed() {
+		return ErrConnectionClosed
+	}
+
+	if cl.Net.Conn == nil {
+		return nil
+	}
+
 	if pk.Expiry > 0 {
 		pk.Properties.MessageExpiryInterval = uint32(pk.Expiry - time.Now().Unix()) // [MQTT-3.3.2-6]
 	}
@@ -544,19 +552,12 @@ func (cl *Client) WritePacket(pk packets.Packet) error {
 		return packets.ErrPacketTooLarge // [MQTT-3.1.2-24] [MQTT-3.1.2-25]
 	}
 
+	nb := net.Buffers{buf.Bytes()}
+	n, err := func() (int64, error) {
 		cl.Lock()
 		defer cl.Unlock()
-
+		return nb.WriteTo(cl.Net.Conn)
-	if cl.Closed() {
+	}()
-		return ErrConnectionClosed
-	}
-
-	if cl.Net.Conn == nil {
-		return nil
-	}
-
-	nb := net.Buffers{buf.Bytes()}
-	n, err := nb.WriteTo(cl.Net.Conn)
 	if err != nil {
 		return err
 	}