Use set_request_property instead of subscriber to improve performance

According to https://github.com/Pylons/pyramid_cookbook/blob/master/auth/user_object.rst

`add_subscriber` cause connect to db even in serving static file.
`set_request_property` only make add_mongo_db be called when you access the property, `mongodb` here.
For Pyramid 1.4, use `add_request_method` should be better. But I don't know whether this package support 1.4 or not.