Move SQL queries from Finalizer into functions in exposure-lib

Several SQL queries accessing exposure data still exist in Finalizer. They should all be encapsulated into function in exposure-lib. These functions are:

• Retrieving buildings within a country boundary
• Retrieving country assets
• Retrieving reference entities for a Quadkey
• Copying from reference assets into assets
• Retrieving all buildings within a tile
• Retrieving reference assets for a reference entity
• Retrieving the number of buildings in a tile
• Multiplying the asset values of an entity by a constant factor

\rfc @tara @laurens