'dropRequest' 事件

• request <http.IncomingMessage> HTTP 请求的参数，如它在 'request' 事件中
• socket <stream.Duplex> 服务器和客户端之间的网络套接字

当套接字上的请求数达到 server.maxRequestsPerSocket 的阈值时，服务器会丢弃新的请求并触发 'dropRequest' 事件，然后将 503 发送给客户端。

• request <http.IncomingMessage> Arguments for the HTTP request, as it is in the 'request' event
• socket <stream.Duplex> Network socket between the server and client

When the number of requests on a socket reaches the threshold of server.maxRequestsPerSocket, the server will drop new requests and emit 'dropRequest' event instead, then send 503 to client.