Update to accommodate latest version of locust

[jayudey-wf]

First off I'm a big fan 😄 , this is my attempt at making the library compatible w/ the most recent versions of locust

Problem

Swarm doesn't work with the latest versions of locust, #26

Solution

Update to handle some of the architectural changes that came with locust

• item one
  • previously it looks like the 'master' was passing in the total number of clients and hatch rate to the workers
  • currently it looks like that information is passed down via the userClassesCount and the total number of workers is incremented from there
• item two
  • for the initial ready check locust is expecting a data field where previously it wasn't (this prompted the changes to the visitor and messages class)

this was heavily influenced by reviewing what was done in locust4j

[anhldbk]

@jayudey-wf Thank you for this MR. Gonna have a look at it.

[anhldbk]

@jayudey-wf LGTM. Thank you!