CBG-3869: Fix losing user xattr on import feed for docs with no sync data

[gregns1]

CBG-3869

• Hooked rawUserXattr to the output of parseXattrStreamData
• Changed the rawXattr return to be nil (in event of no sync data) and moved the rawUserXattr to the correct return value
• Testing for functionality of importing a doc with user xattr defined with no sync data on it through the import feed. In addition have part of the test that writes a doc with array of channels to test that functionality of that.
• Assert that the above docs are assigned the channels correctly

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/2377/

[torcolvin]

LGTM, some tiny test improvements for future debugging.

It'd be cool to have this tested outside RestTester as a unit test on this function. I'm working on a test function to construct a raw body for UnmarshalDocumentSyncDataFromFeed in CBG-3835

[torcolvin]

LGTM, some tiny test improvements for future debugging.

It'd be cool to have this tested outside RestTester as a unit test on this function. I'm working on a test function to construct a raw body for UnmarshalDocumentSyncDataFromFeed in CBG-3835