feat: Multidatasource

[farfromrefug]

This is a new datasource which kind of work like the PackageManagerDataSource but with any local datasource.
It allows to handle multiple datasources in a better way than order or combine.

A few notes:

• i am not really happy with this. The idea is not go through data sources which are already cached. I would prefer some kind of difference method between the 2 vectors but could not get it to work.
• i have one lasting issue with it for which i might need your help. The only issue i see is when used for hillshade like i do here https://github.com/Akylas/mobile-sdk/blob/master/scripts/android-dev/app/src/main/java/com/akylas/cartotest/ui/main/SecondFragment.java#L145= My issue is that for some reason the hillshade render is broken on "overzoom"

while debugging i can see that the over tiles are correctly loaded, if i break i even see that first the render is ok

But then as it finishes rendering the map it then get broken

If i change the hillshade layer to not use the MultiTileDataSource but the MBTilesTileDataSource directly then it renders correctly(like the first screenshot).

hope you can help

[farfromrefug]

about my rendering issue it seems to got fixed with my latest commits. Not sure why though :s