refactor(listener) refactor listener related code

[xiang90]

Remove duplicate code around creating http listener.
Start to listen incoming http requests just before serving them.

[xiang90]

@unihorn @philips

[xiang90]

This is the first step towards fixing the TLS issue.

[philips]

/cc @bcwaldon

[bcwaldon]

lgtm if it works

[xiang90]

@unihorn @bcwaldon All the tests passed except the TLS one I am fixing. More prs are needed to fix that one.

[freeformz]

Inquiring minds want to know which TLS issue?

[xiang90]

@freeformz We have a TLS functional test that keep on failing https://drone.io/github.com/coreos/etcd/91
The root cause are:

1. bad timeout assumption for first time TLS handshake
2. bad order of starting etcd internal service

It is not a huge issue for normal use case, but in the test case we start all the TLS nodes together on the same machine.