Switch to pulling way_area from tags hstore instead of assuming it to be a column

[iandees]

While running through the vector-datsource import process for #2001 earlier this week, I ran into a problem where the way_area column was missing:

DataFetchException: column "way_area" does not exist
LINE 104:     way_area,
              ^

Talking to Jochen at osm2pgsql (osm2pgsql-dev/osm2pgsql#1589), it sounds like this was a regression/change in how the way_area special tag is handled around osm2pgsql 1.3.

Since the previous behavior was to add way_area to both the "tags" hstore column and as a distinct column, we should change vector-datasource to pull it from the tags hstore so it's compatible with newer and older osm2pgsql versions.