Allow multiple sequential rpc requests

With a same client, I would like to launch mulitple RPC queries in a row:

```php
$rpc->addRequest(serialize($msg), 'server', $action);
$replies = $rpc->getReplies();

// then
$rpc->addRequest(serialize($msg), 'server', $action);
$replies = $rpc->getReplies();
```

Without this patch, $this->requests is increasing without ever decreasing, preventing the second series of requests to never end, always waiting for new replies.

In theory, I can just launch new RPC client, but why lose an open connexion?