ppf: add retrievals field so we know whether an url is new

use

sqlite3 urls.sqlite "alter table uris add retrievals INT"
sqlite3 urls.sqlite "update uris set retrievals=0"

dbs.py

@@ -19,6 +19,7 @@ def create_table_if_not_exists(sqlite, dbname):
 			check_time INT,
 			error INT,
 			stale_count INT,
+			retrievals INT,
 			added INT
 			)""")