Close cnxPool when closing a Client (#494)

diff --git a/pulsar/client_impl.go b/pulsar/client_impl.go
index ee8ff50..8db3028 100644
--- a/pulsar/client_impl.go
+++ b/pulsar/client_impl.go
@@ -188,6 +188,7 @@
 
 func (c *client) Close() {
 	c.handlers.Close()
+	c.cnxPool.Close()
 }
 
 func (c *client) namespaceTopics(namespace string) ([]string, error) {