Clean up some ordering and strata edge cases in Phrequent

Summary:
Ref T3569. Two issues:

1. Since sort() is not stable, instantaneous events (ending on the same second they start) would sometime sort wrong and produce the wrong results. Guarantee they sort correctly.
2. Because events can end at any time, there are some additional special cases the algorithm didn't handle properly. Draw a bunch of ASCII art diagrams so these cases work properly.

Test Plan:

• No more fatal when tracking an object for the first time.
• Unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: skyronic, aran

Maniphest Tasks: T3569

Differential Revision: https://secure.phabricator.com/D7350