CBG-4383 handle no revpos in attachment block

[torcolvin]

CBG-4383 handle no revpos in attachment block

I'm not sure whether to implicit treat no revpos as 0 is OK or if I should avoid this entirely. TestForEachStubAttachment has code that relies on checking revpos.

I'm also not sure if we should output revpos in the output of GET /ks/doc but probably for pre 3.x compatibility.

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/2816/