CBG-5022: move rev cache locks into defer where possible

[gregns1]

CBG-5022

• I haven't changed the performEviction function given that has been changed in different PR
• Please review carefully and watch for anything I may have missed

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/0000/

[Copilot]

Pull request overview

This PR refactors lock handling in the LRU revision cache implementation by moving explicit lock.Unlock() calls into deferred statements, improving code safety and maintainability by ensuring locks are always released even if early returns occur.

Key Changes:

• Converted manual lock releases to defer statements across multiple cache methods
• Extracted cache insertion logic into a new helper function upsertDocToCache
• Simplified control flow by removing early returns after lock releases