Functions `get_district_built_area()` and `get_district_tiles_with_built_area()` use sequential scan

The execution of the get_district_built_area() and get_district_tiles_with_built_area() functions takes very long due to a sequence scan in the SQL queries. The queries need to be optimized to use index scans.

\rfc @gislars @pdlmora @laurens @tara

Edited by Danijel Schorlemmer