Bug 893910 - Part 3: Review comments and followups.

* Review comment: Move reset check later, add doc, and add reset on
success and failure.
* Followup: Add tests for failure count reset on upload success or hard
failure.
* Followup: Add comment to ambiguous case as per IRC discussion w/
rnewman.
* Review comment: Use defaults in upload reset time tests.
* Review comment: Clarify comment describing when we reset the failure
count.
* Review comment: Move failure reset into clear method (rather than
setting -1 by hand).