Clean up some ordering and strata edge cases in Phrequent

Summary:
Ref T3569. Two issues:

  # 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.
  # 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