Update dispatch on mysql command sleep

The handling is now matching the MySQL. It will raise error UNKNOWN
COMMAND when receiving command COM_SLEEP from clients.

server/conn.go

@@ -1335,11 +1335,6 @@ func (cc *clientConn) dispatch(ctx context.Context, data []byte) error {
 	}
 
 	switch cmd {
-	case mysql.ComSleep:
-		// TODO: According to mysql document, this command is supposed to be used only internally.
-		// So it's just a temp fix, not sure if it's done right.
-		// Investigate this command and write test case later.
-		return nil
 	case mysql.ComQuit:
 		return io.EOF
 	case mysql.ComInitDB:

server/conn_test.go

@@ -349,12 +349,6 @@ func TestDispatch(t *testing.T) {
 	userData = append(userData, 0x0)
 
 	inputs := []dispatchInput{
-		{
-			com: mysql.ComSleep,
-			in:  nil,
-			err: nil,
-			out: nil,
-		},
 		{
 			com: mysql.ComQuit,
 			in:  nil,
@@ -468,12 +462,6 @@ func TestDispatchClientProtocol41(t *testing.T) {
 	userData = append(userData, 0x0)
 
 	inputs := []dispatchInput{
-		{
-			com: mysql.ComSleep,
-			in:  nil,
-			err: nil,
-			out: nil,
-		},
 		{
 			com: mysql.ComQuit,
 			in:  nil,