Improve permission-conditions

[thomasyip]

https://github.com/beedesk/django-trusts/blob/master/docs/source/index.rst#permission-conditions
https://github.com/beedesk/django-trusts/blob/master/trusts/backends.py#L87

Currently, permission_condition_met assume the permission func is a simple lambda. The code should be improved such that it can be filtered on the query set.

The rest of the code (and API) can be improved to reduce the number of hits to DB.