Bug fixes with updating interval_start

Summary: interval_start should update even when number fetched equals max_entities_per_fetch, also typo fix: check for colliding timestamps should be that timestamp_first = timestamp_last...

Test Plan: Using new fetch_entities.py in tmp_analytics on the analytics machine, ran test.py and it works gloriously!

Reviewers: jace

Differential Revision: http://phabricator.khanacademy.org/D3099