fix(api): serve until the "acceptconnections" job

This fixes a bug that I encountered when using socket activation with
docker 0.8.1. When running the first `docker run` it would return:
"create: command not found".

The root cause was the socket activation code path was starting to
listen before the "initserver" job had finished. This meant that the
"create" handler hand't been registered yet leading to the command not
found error.

In log format it looks like this:

```
[/var/lib/docker|9d2e78e9] +job initserver()
2014/03/01 04:05:35 Listening for HTTP on fd ()
[/var/lib/docker|0d71c177] +job create()
create: command not found
[/var/lib/docker|0d71c177] -job create()
[/var/lib/docker|0d71c177] +job acceptconnections()
[/var/lib/docker|0d71c177] -job initserver() = OK (0)
```

To fix the issue select on the activationLock and block until the
"acceptconnections" job has ran.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
Merge pull request #4398 from philips/fix-socket-activation

fix(api): serve until the "acceptconnections" job