Make review-requests API properly handle ambiguous times.

Review Request #318 — Created Jan. 16, 2017 and updated — Latest diff uploaded

Review Board
Because we live in a silly world where some people arbitrarily move their
clocks back and forth by an hour, timestamps, even those containing timezone
information, may not be unique. In particular, this could cause problems when
fetching the `review-requests` resource using the time-limiting fields with a
timestamp that exists during the hour after the transition from
daylight-savings time to standard time.

This change fixes things up to catch the error and return a reasonable error
via the API, rather than a 500 and a traceback. This also fixes it up so
invalid timestamp strings return an error rather than getting silently ignored.

Testing done:
Ran unit tests.

Reviewed at