Make Client responsible for closing its conn

Prior to this, closing client connections was done in a separate
step outside of `client.Close()`.

However it makes sense to move this responsibility to Client, the
higher-level abstraction that already contains conn in its internal
state.

This change does not alter the actual closing sequence:
consumers/producers are closed first and then the client connection.

Closes #36.

client.go

@@ -132,8 +132,8 @@ func (c *Client) Producer(cfg *rdkafka.ConfigMap) (*Producer, error) {
 	return c.producer, nil
 }
 
-// Close closes c's underlying producers and consumers. Calling Close on
-// an already closed client will result in a panic.
+// Close closes producers, consumers and the connection of c.
+// Calling Close on an already closed client will result in a panic.
 func (c *Client) Close() {
 	for cid := range c.consumers {
 		// TODO(agis): make this blocking
@@ -143,4 +143,6 @@ func (c *Client) Close() {
 	if c.producer != nil {
 		c.producer.Close()
 	}
+
+	c.conn.Close()
 }

server.go

@@ -55,10 +55,9 @@ func NewServer(ctx context.Context, manager *ConsumerManager, timeout time.Durat
 }
 
 func (s *Server) handleConn(conn net.Conn) {
-	defer conn.Close()
-
 	c := NewClient(conn, s.manager)
 	defer c.Close()
+
 	s.clientByID.Store(c.id, c)
 	defer func() {
 		s.clientByID.Delete(c.id)