Send -1 as version when sending client_ready message after test has stopped

[stephen-harris]

This fixes a bug where after stopping the test the workers disappear.

Locust version: 2.0.0b3 Locust4j version: master

Steps to reproduce

• Stop a running test
• Expected: workers to remain 'ready'. Actual: ready workers drop to 0. Logs contain:

  worker_1 | 21:34:30.713 [Thread-2receive-from-client] DEBUG com.github.myzhan.locust4j.runtime.Runner - Recv stop message from master, all the workers are stopped master_1 | [2021-08-14 21:34:30,715] 1e6ea2aa04b2/INFO/locust.runners: Removing 0fff9756d6ca_3bcbb4e6b4a4fb9bd4e00c49e9e2da23 client from running clients master_1 | [2021-08-14 21:34:30,715] 1e6ea2aa04b2/ERROR/locust.runners: An old (pre 2.0) worker tried to connect (b0aa3c23ab9b_c2f230bf39580a6048734e537229e70b). That's not going to work. master_1 | [2021-08-14 21:34:30,715] 1e6ea2aa04b2/ERROR/locust.runners: An old (pre 2.0) worker tried to connect (0fff9756d6ca_3bcbb4e6b4a4fb9bd4e00c49e9e2da23). That's not going to work.

The "-1" version is included in the client_ready message sent when the worker first starts, but was missing when this message was sent after a test has been stopped.

[myzhan]

Thanks!