Refactor SecurityFilter (Whoosh 2.5 compatibility).

SecurityFilter used to keep filters in lists, converting them to queries when subqueries property was accessed.
This worked with Whoosh 2.4.1, but failed with Whoosh 2.5. (Whoosh 2.5 adds additional caching, which checks
SecurityFilter.__hash__ before SecurityFilter.subqueries is accessed).

Refs: #741


git-svn-id: https://svn.apache.org/repos/asf/bloodhound/trunk@1559080 13f79535-47bb-0310-9956-ffa450edef68