CBG-4206: read/write attachments to global sync xattr

[gregns1]

CBG-4206

• Read attachments form new _globalSync xattr
• If not defined there but in sync data instead, code will read from there instead
• All new writes will have attachments moved to _globalSync
• Kept in memory storage of attachment metadata in sync data so essentially global xattr is only used at read/write time
• Wanted to remove the attachments json property from sync data struct (like what was done with HLV in sync data) but this broke migration from inline sync data to xattr so had to keep it in.

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/SyncGateway-Integration/2675/
• 5th Sept - Coverage test: https://jenkins.sgwdev.com/job/SyncGateway-Integration/2677/
• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2683/

[bbrks]

Looks good, just a question about the new test

[bbrks]

Is it worth extending this test to include a document update as well, to ensure we move the persisted xattrs over as well?

[gregns1]

Good shout! Extended this test but also added new test for update to doc with attachment meta in sync data.