WIP: update locust to run ontop of requests > 1.0

notes:
- use response.elapsed for measuring response times
- update comment to reflect safe_mode being removed from requests 1
- replace config (which doesnt exist in 1.2)
- remove on_pre_request method as that doesnt exist anymore
- requests.Session objects don't take args or kwargs anymore. Set the
  attrs on the instance after instantiation
- as this is a backwards incompatible change, bump the major version number
- requests.request param stream -> prefetch