Add functionality to run listener functions for custom_messages concurrently

[samuelspagl]

This MR adds the functionality to Runners.register_message to register a concurrent function.

Issue #2608 explains this PR in more detail.

[samuelspagl]

Hi @cyberw,

I implemented a possible way to register concurrent custom_message functions.

I also thought about adding a new variable and function to the Runners class. The variable would store all of the concurrent running greenlets, and the function could join / or kill them.

What do you think about it?

[cyberw]

Looks nice! Just fix the linting and I'll merge!

[cyberw]

Oh yea, add something to the documentation as well (mainly examples/custom_messages.py, but mention it in running-distributed.rst as well)

[samuelspagl]

@cyberw, pipeline seems to be fine. I added documentation and adjusted the example.

[cyberw]

Thanks! I'll make some small adjustments to the example, but no need to do it in this PR.

[samuelspagl]

Thanks a lot! Do you have a time estimate for the release? No pressure, just for my planning :)

[cyberw]

I dont have any release planned, but every merged pr becomes a prerelease. You can install it using pip install locust==2.24.2.dev8

[samuelspagl]

Ah nice, didn't know that. Thanks a lot!

[cyberw]

I tried it out and tweaked the implementation a little. I hope this change doesnt break anything for you :) 061552f