CBG-5049: switch caching feed mutex to exclusive only mutex

[gregns1]

CBG-5049

• Switches caching feed mutex up to exclusive mutex
• We will iterate on perf tests for this and if regresses we will revert

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Dependencies (if applicable)

• Link upstream PRs
• Update Go module dependencies when merged

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/203/

[Copilot]

Pull request overview

This PR converts the change cache's locking mechanism from a read-write mutex (sync.RWMutex) to an exclusive mutex (sync.Mutex). This change simplifies the locking strategy by treating all operations uniformly, potentially addressing concurrency issues at the cost of some read concurrency. The team plans to evaluate performance impacts and may revert if there are regressions.

Key Changes:

• Replaced sync.RWMutex with sync.Mutex in the changeCache struct
• Updated read lock calls (RLock()/RUnlock()) to exclusive lock calls (Lock()/Unlock())

[bbrks]

As discussed will just park this until we have a good perf baseline to measure against then decide whether to do it or not.