implement configurable blocking of requests when the server is too busy: issue #2722
loadgen fixes and improvements:
  * defend against double callback invocation in wsapi_client for network errors
  * display average request latency in loadgen output
  * fix unhandled exception when cert_key API fails
  * handle 503 errors at any point, not just in auth responses.
bump maximum_event_loop_lag to 40ms
fix double inclusion of 'shutdown'
a simple script to create test users for load generation
fix error introduced when implementing common error handling - fake_verification API does not return json, and that's ok
fix inverted logic for disabling request throttling
Merge pull request #2743 from mozilla/issue2722

refuse requests when the server is too busy