MDEV-31344: fast status variable lookup

[dmitry416]

Summary

• Added a hash table for status variables
• Now the variable is searched first in the hash table, and if it is not found, it is searched using the old path.

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[gkodinov]

Thank you for your work. This is a preliminary review.

Please make sure the buildbot servers compile and run tests successfully to completion.

Also, the whole diff needs work:

• the hash should not be filled in only at init time. It should also be filled in when status variables array version is increased (e.g. load a plugin etc).
• hash access should be protected under LOCK_all_status_vars.
• you need a test that covers the change as much as possible.
• to be able to test you might need some new status variable counters to observe the hash usage vs scans.

I would start with an exact design document of what the diff is trying to do. Then I'd add regression test to ensure that all the functional and non-functional requirements are tested. And then compare the implementation to the design spec and the tests and make sure it delivers.