Allow master node to supply data to worker nodes directly

[nathan-beam]

Is your feature request related to a problem? Please describe.

Pretty much a duplicate of #1506, but it would be extremely beneficial for locust to be able to communicate data between nodes easily. An example that you're probably familiar with is needing to retrieve user accounts from a database for login, and trying to coordinate that between several worker nodes so that the same users are not logged in across multiple workers.

Describe the solution you'd like

Adding hooks to allow communication between nodes would be ideal. I'm not sure exactly what the best solution would be but allowing the user to define custom messages and handlers for the greenlets seems like it'd be easy enough to implement and massively improve locust's ability to coordinate itself in distributed mode.

Describe alternatives you've considered

The currently accepted workarounds (e.g. mongodb or a REST api) are much less than ideal, especially for something like a docker environment where each additional component must be installed and set up regularly.

Additional context

I'm available to give this a shot, but I wanted to make sure A) it has value and B) the best implementation from those who are more experienced on this project.

[cyberw]

Absolutely, go for it! Try to build upon the existing zeromq communication.