Fix memory leak in timeutils/cache (syslog-ng#4332)

[meeusr]

Each time a worker thread terminates, the 2 allocated tznames are leaked. Solution is to explicitly deinit the cache from the stop function of the thread.

[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
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do nothing -> CI won't start)

[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
"add to whitelist" add author of a Pull Request to whitelist (globally, be careful, it means this user can trigger kira for any PR)
do nothing -> CI won't start)

[github-actions]

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[bazsi]

Ok to test.