MDEV-30418 : Setting wsrep_slave_threads causes thread hang

[janlindstrom]

• [x] The Jira issue number for this PR is: MDEV-30418

Description

Problem was that wsrep was disconnected and new slave threads tried to connect to cluster but failed as we were disconnected state.

Allow changing wsrep_slave_threads only when wsrep is enabled and we are connected to a cluster. In other cases report error and issue a warning.

Release Notes

TODO: What should the release notes say about this change? Include any changed system variables, status variables or behaviour. Optionally list any https://mariadb.com/kb/ pages that need changing.

How can this PR be tested?

TODO: modify the automated test suite to verify that the PR causes MariaDB to behave as intended. Consult the documentation on "Writing good test cases".

If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.

Basing the PR against the correct MariaDB version

• [ ] This is a new feature and the PR is based against the latest MariaDB development branch.
• [x ] This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• [x ] I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• [ x] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

[robinnewhouse]

This looks good to me! I just added one question, mainly for my own curiosity.