Set password value and expiration atomically

Redis has a `SETEX` (http://redis.io/commands/SETEX) method,
which does both setting the value and setting the expiration
atomically.

snappass/main.py

@@ -48,8 +48,7 @@ def check_redis_alive(fn):
 @check_redis_alive
 def set_password(password, ttl):
     key = uuid.uuid4().hex
-    redis_client.set(key, password)
+    redis_client.setex(key, ttl, password)
-    redis_client.expire(key, ttl)
     return key