conn: do not lock sending when closing (#70)

The sending mutex may be blocked due to the underlying conn blocking. If
that happens then we can't call close since close also attempts to hold
the sending mutex. Sending mutex is only used for serializing sends and
doesn't protect the fields close reads/writes. I believe we introduced
locking it so we would return ErrClosed. This commit instead introduces
a check in send which rechecks if we have since closed while attempting
to send.

Test Plan: expanded the test coverage

conn.go

@@ -166,9 +166,7 @@ func (c *Conn) SendResponse(ctx context.Context, resp *Response) error {
 }
 
 func (c *Conn) close(cause error) error {
-	c.sending.Lock()
 	c.mu.Lock()
-	defer c.sending.Unlock()
 	defer c.mu.Unlock()
 
 	if c.closed {
@@ -249,6 +247,17 @@ func (c *Conn) send(_ context.Context, m *anyMessage, wait bool) (cc *call, err
 	c.sending.Lock()
 	defer c.sending.Unlock()
 
+	// double check the error isn't due to being closed while sending.
+	defer func() {
+		if err != nil {
+			c.mu.Lock()
+			if c.closed {
+				err = ErrClosed
+			}
+			c.mu.Unlock()
+		}
+	}()
+
 	// m.request.ID could be changed, so we store a copy to correctly
 	// clean up pending
 	var id ID