apiserver: avoid data race in shutdown

[davecheney]

Fixes LP 1465115

Ensure that the listening socket is always the first thing that is closed
when Server.Kill or Stop is issued. This avoids a race where requests in
fly may increase the waitgroup count while the final wg.Wait() defer is in
effect.

(Review request: http://reviews.vapour.ws/r/2072/)

[davecheney]

@rogpeppe PTAL

[davecheney]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Does not match ['fixes-1470601']
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/3852

[davecheney]

$$JFDI$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Does not match ['fixes-1470601']
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/3853

[davecheney]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Generating tarball failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/3858

[davecheney]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju