MOB-2213: Add information about poi count to map CRUD API

I've spoke with Zach yesterday about POI count for list of maps on wikia. We do not display it currently on Special Maps and it's not a required feature for the first release of VE support for maps. Of course It's an interesting add-on. There are at least couple of ways how to get this information. For example we could add `poi_count` column to map table and update it every time the poi for this map is added or deleted (we do something similar with `updated_on` column now. By doing that we would avoid `join` for every GET request. 

My proposal is to move this feature to next release and rethink the usage, benefits, requirements and implement it  the way that meets them all and is good for overall performance.