Call wait function in on_start

[williamlhunter]

Request

I'm not sure if this is a design choice, but I would find the change useful. To accurately simulate user-behavior, I have to call self.wait in my on_start function. I think most people would prefer this, but I could be wrong.

Description of issue

Loust not calling wait_function after on_start().

Expected behavior

On start is called, then the locust waits before executing the first task in the task_set.

Actual behavior

wait_function is not called

Environment settings

• OS: windows
• Python version: 3.6.3
• Locust version: 0.10.0