authenticator: Cleanup around DB usage

Don't rollback the database when we don't use transactions (they are
useless here, with single select being done to refresh again and again).

Also, don't recycle the cursor, clean it up (it is reportedly better for
the DB).

src/master/authenticator/authenticator.py

@@ -40,26 +40,23 @@ with open(sys.argv[1]) as f:
 	config_data.readfp(f, sys.argv[1])
 
 db = None
-cursor = None
 cred_cache = {}
 lock = Lock()
 
 def openDB():
 	global db
-	global cursor
 	db = psycopg2.connect(database=config_data.get('main', 'db'), user=config_data.get('main', 'dbuser'), password=config_data.get('main', 'dbpasswd'))
-	cursor = db.cursor()
 
 openDB()
 
 def renew():
 	print "Caching auth data"
+	cursor = db.cursor()
 	cursor.execute('SELECT name, passwd, mechanism, builtin_passwd, slot_id FROM clients')
 	lines = cursor.fetchall()
 	global cred_cache
 	# This should replace the whole dictionary atomically.
 	cred_cache = dict(map(lambda l: (l[0], l[1:]), lines))
-	db.rollback()
 	print "Caching done"
 
 renew()