EZP-23206: prevent parallel publishing of versions of one content

> Status: Ready for review
> See http://jira.ez.no/browse/EZP-23206

Publishing multiple versions of the same content in parallel, with or without asynchronous publishing, can lead to multiple versions with the published state. Furthermore, if a language is added to each draft, and all are published at the same time, the published version will very likely lack several languages.

This patch fixes it by preventing the asynchronous publishing queue reader from picking a version for publishing if another version of the same content is already being published.

Sequence is maintained by ordering first on the queuing date, then on the version number.

It has been tested using [this gist](https://gist.github.com/bdunogier/b47784a14a9cc946a0c6) that publishes a folder, adds one version per given locale, and  sends everything for publishing in parallel. Run `export EZ_USE_AP=1` before running the script to pass the items to asynchronous publishing (still needs to be enabled).

![multipublish](https://cloud.githubusercontent.com/assets/235928/3737208/cee98a9a-1734-11e4-8819-77260a58af64.PNG)