add CVMFS_LOW_SPEED_LIMIT parameter

Make the threshold for a stale connection configurable, increase default from 100B/s to 1kB/s ([CVM-718](https://sft.its.cern.ch/jira/browse/CVM-718))
I'd suggest to make stubs fail by default... otherwise one might forget to replace the stub with an actual implementation... the perfect snake oil ;)