Use SafeRequestStore in the DB LB
What does this MR do and why?
In #339515 (closed) it was found that specs can fail when run in a certain order, possibly due to the use of RequestStore (which can leak between tests) instead of SafeRequestStore.
This commit changes the load balancer to use SafeRequestStore, ensuring we don't leak any data between tests.
MR acceptance checklist
Quality
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Quality checklist confirmed 
Performance, reliability, and availability
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Performance, reliability, and availability checklist confirmed 
Documentation
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Documentation checklist confirmed 
Security
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Security checklist confirmed 
Deployment
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Deployment checklist confirmed 
Edited  by Yorick Peterse