DB Overpass: compiler, add db
Merge remote-tracking branch 'origin/db-postgresql-as-base-class' into db-overpass

Conflicts:
	pgmapcss/compiler/compile_sql.py
DB/Overpass: resolve set statements
DB/Overpass: improve db functions

- fix union in queries
- build either linestrings or polygons
- bugfixes
DB/Overpass: First attempt in implementing multipolygons
DB/Overpass: multipolygons: construct from non-closed linestrings
DB/Overpass: bugfix arranging way areas
DB/Overpass: handle tag-less multipolygons (use tags of outer ways)
DB/Overpass: Bugfix, loading meta data of objects
DB/Overpass: include (all) areas which are larger than bbox
DB/Overpass: Bugfix setting ID for areas
DB/Overpass: re-implement objects_by_id()
DB/Overpass: add option to set a different overpass api url
DB/Overpass: handle error message when areas are not available in overpass api
DB/Overpass: Bugfix, multipolygons: not all outer ways in bbox
Merge branch 'db-overpass' of github.com:plepe/pgmapcss into db-overpass
Compile DB selects: change list of conditions to list instead of set
DB/Overpass: first compile to tokens, then to strings; also compile other conditions
DB/Overpass: merge conditions if they build subsets
DB/Overpass: merge conditions if one of them queries for key