for transport that are using timeout connections, we set the
maxIdleConnsPerHost to -1. The default transport does not clear
the timeout for the connections it sets to be idle. So the connections
with timeout cannot be reused.
Dial timeout is set shorter because
1. etcd is supposed to work in good environment, and the new value is long
enough
2. shorter dial timeout makes dial fail faster, which is good for
performance
