apply back-off logic to utrace, will prevent requests for 60s after a failure

@sjcorbett noted in #156 that utrace lookups may be slowing down his startup.  i could not replicate that; utrace lookups have negligible impact for me, and caching and timeouts are in operation, but in case offline mode / poor network was responsible, this prevents utrace from trying lookups for 60s after a failure.  @sjcorbett does this help?