Adjust HgResumeTransport behavior on two edge cases

Fixed two questionable behaviors while searching the code for
why some users of LanguageDepot have seen no-op merges happen
during their Send/Receives

* Count a ServiceUnavailable response as a timeout when
  retrieving revisions instead of logging and returning
  an empty map.
* If the server indicates that changes happened in the middle
  of a pull operation rebuild the local cache of revisions before
  restarting the pull.
Do not retry when ServerUnavailable response is received

This addresses the review comments
* break and throw if ServeUnavailable is received during the
  retrieval of revisions from the server
* remove test which checked for that behavior
Merge pull request #9 from sillsdev/ResumableFixes

Adjust HgResumeTransport behavior on two edge cases