slack-15.0: forward ports slack specific v14 PRS fix

[vmogilev]

Description

forward ports slack specific v14 PRS fix: #177

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[timvaillancourt]

@vmogilev return nil makes the client unaware the repairReplication never happened. Should we return an error to the RPC caller that explains what happened?

[timvaillancourt]

@vmogilev _isSetReplicationSourceLockedRunning is a concurrently-access struct field, so it needs to be read/updated under a mutex or this causes a race. This field is now protected by the mutex designed for read/writing _ vars in the struct

I attempted to add a unit test so the race detector would prove this to be true, but it's a complicated test to write, so I have deferred this

[timvaillancourt]

Adding more reviewers as I've now changed code