Petra: improve dynamic management of guard units
Summary:
Petra will not add guards when it has a low population, and will remove any citizen-soldier guards so they can be used for other tasks.
Petra will also use a healer queue for training healer guards, and this is done mainly so that Petra doesn't clog the villager queue with healers, as villages should always have a higher priority. Also this queue could be used in the future for training healer units for other purposes.

Patch by Sandarac

Differential Revision: https://code.wildfiregames.com/D165