consider enable dialClient keepalive again

[disksing]

Previously, we have disabled keepalive in #969 due to #971. I think if we can reuse clients, there won't be too many connections, so we should enable it again.

[disksing]

@siddontang

[siddontang]

I think we misuse the client:

resp, err := dialClient.Get(fmt.Sprintf("%s%s", cURL, healthURL))
if resp != nil {
		resp.Body.Close()
}

we should read all body before close it.