ht: improve latency checking
ht: improve latency checking

Two minor improvements: 1. Increase the fixed timeout from 2 to 3 minutes
as sampling 100 requests for high accuracy might take some time.
2. If not enough responses could be sampled report the failed limits anyway.
The numbers hav low statistical significance but they are informative
and shouldn't be suppressed.

vdobler/ht,vdobler/ht,vdobler/ht,vdobler/ht