Drop support for Locust 0.x, and make use of SequentialTaskSet optional, etc

[cyberw]

In #80 I said:

For a new major version I would love to drop Locust 0.x support and maybe put everything in one task (insted of @task:s in a SequentialTaskSet - task sets are really an advanced feature, and a lot of people get confused when using them :)
And maybe optionally add catch_response=True and with-blocks.

@thilp said:

I must admit I’ve not kept myself up to date with Locust best practices, so I take your word for it. If I remember correctly, we are using SequentialTaskSet to attribute different weights to scenarios. How would you achieve the same flexibility with a single task, have Transformer hardcode the load-balancing code directly inside the task? Or do you simply not use weights?

Perhaps we can also continue this discussion in a dedicated issue, since this will likely outlive this PR 🙂

Yes, lets do that :)

I would suggest discarding the support for weighting (as it is much more user friendly to build upon a generated script where every request as a single line) or leave the current SequentialTaskSet-approach as an option (preferably not the default).

I think we should also rename a some things, as recorded requests is the basic building block, not tasks (like Contract.OnTask to Contract.OnRequest)

Another useability improvement I would like to suggest is that a plugin should be able to return None, and that should simply ignore the request (instead of causing an exception).