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
update entire row when stagin email
Show the "This account cannot be verified" tooltip if the user enters a verification password < 8 chars.
Merge pull request #2942 from mozilla/issue_2941_stage_update_row

update entire row when staging email
Explain why a 401 response is synthesized if a password is too short for email verification.
Merge pull request #2944 from mozilla/issue_2943_short_verification_password

Show the "This account cannot be verified" tooltip if the user enters a verification password < 8 chars.
Merge pull request #2743 from mozilla/issue2722

refuse requests when the server is too busy