CBG-4309: update cbgt to address lack of janitor kick during rollback

[gregns1]

Uptake a fix in cbgt from MB-64885 on a special branch of cbgt that contains this fix on top of v1.3.9 - https://github.com/couchbase/cbgt/tree/sg_v139

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/2916/