CBG-4562 do not return invalid sessions with GET

[torcolvin]

CBG-4562 do not return invalid sessions with GET /db/_user/{username}/_session

• Fix the behavior of GET /db/_session to return not found when it encounters an invalid session by just returning NotFound when the session is present but invalid.
• Change the behavior of GetSession to return an error when a session is not found to make the behavior of the function more obvious to avoid having to check for session == nil when err == nil.

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

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2988/

[github-actions]

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