Fix creating SQL for ecoservice when searching stewardship

Hstore parameters are converted by Psycopg2, which wasn't being used
by the ecobenefits view.  Now instead of just quoting strings, we let
Psycopg2 handle all of the sql parameter conversion through the
non-standard mogrify method
Merge pull request #1314 from maurizi/fix-stewardship-search-backend

Fix creating SQL for ecoservice when searching stewardship