Queries for subj_uri and subj_origin are using no index

[Markus Korn]

We have a bunch of queries which are using no index and therefor are much slower than others, this is one example:

Running: engine.find_eventids(TimeRange.always(), [], StorageState.Any, 6, ResultType.MostRecentSubjects)
DEBUG:zeitgeist.sql:Got query:
QUERY:
SELECT DISTINCT id FROM event_view GROUP BY subj_uri ORDER BY timestamp DESC LIMIT 6 ([])
PLAN:
[0, 0, u'TABLE event']
[0, 0, u'TABLE uri USING PRIMARY KEY']

DEBUG:zeitgeist.engine:Found 6 event IDs in 0.571514s
---> MostRecentSubjects: get len(ids)=6 using .find_eventids() in 0.571749s

All quries with subj_uri and subj_origin are affected.