show agent sockets stat. fixed #433

```log
[httpAgent] socket: 3 created, 0 close, 0 timeout, request: 50 requests, 17 req/socket
working sockets registry.npmjs.org:80::: 1
free sockets registry.npmjs.org:80::: 2
[httpsAgent] socket: 0 created, 0 close, 0 timeout, request: 0 requests, 0 req/socket
```