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
Merge pull request #2942 from mozilla/issue_2941_stage_update_row

update entire row when staging email
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
remove no-longer-needed call to shutdown.handleTerminationSignals
Merge pull request #2952 from jrgm/remove-duplicate-shutdown-registration

remove no-longer-needed call to shutdown.handleTerminationSignals
update version.js to run asynchronously and eliminate race conditions in code
Avoid sha pollution if the sha reported in ver.txt is incorrect.
Adding a code version mismatch detection feature. Fixes Issue #2710

Conflicts:
	resources/static/common/js/network.js
Hook up the code version check middleware.

* Use the X-BrowserID-Version header in all WSAPI requests to signal which version of the code is being used.
* Make sure all the /wsapi calls in the tests have X-BrowserID-Version headers
* Update version retreival to occur asynchronously.