fix thingsapi#125: Close database connection after each query

Each query creates a new DB connection.
This change is to close each new connection rather than leaving
them open, to allow for runs which involve large numbers of queries.

things/database.py

@@ -503,7 +503,9 @@ def execute_query(self, sql_query, parameters=(), row_factory=None):
         cursor = connection.cursor()
         cursor.execute(sql_query, parameters)
 
-        return cursor.fetchall()
+        result = cursor.fetchall()
+        connection.close()
+        return result
 
 
 # Helper functions