CBG-3764 create multi-xattr subdoc APIs

[torcolvin]

Update multi xattr subdoc APIs to support multiple xattrs. Where the APIs previously accepted two xattrs (user xattr and sync xattr), continue to allow two xattrs with CBS 7.6 with a CAS check in a loop. If the API previously did not accept two APIs, Couchbase Server 7.2 will return an invalid argument error if multiple xattrs are not specified.

Possible enhancements:

• enhance rosmar to be able to toggle on BucketStoreFeatureMultiXattrSubdocOperations to be able to test without running two versions of couchbase server
• Writing a check in all xattr APIs to make sure there are only 1 or 2 supported xattrs if using CBS < 7.6 which provides a bit easier message than one.
• Write a check in all xattr APIs to make sure that xattrKeys are never empty string. This was a late fix in integration tests from a somewhat opaque gocb error message. Alternatively, error in rosmar if any xattrKeys are empty.
• Move ErrXattrNotFound and ErrXattrPartialFound to sg-bucket and return in rosmar?

Things to look for in the PR:

• Did I remove any operations that should be retried to work a single time?
• Should I rename any functions to make it more clear which is the function that will do a cas based retry and which does the actual operation?
• Double checking any cases where I change logging for redaction OR error wrapping.

couchbaselabs/rosmar#25
couchbase/sg-bucket#112

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,CBS-7.2.2 https://jenkins.sgwdev.com/job/SyncGateway-Integration/2366/
• GSI=true,xattrs=true,CBS-7.6.1-3122 https://jenkins.sgwdev.com/job/SyncGateway-Integration/2364/

[adamcfraser]

A few minor comments/questions but otherwise looks good to me.

[adamcfraser]

A helper function to build the xattr keys wouldn't be a bad idea, since we're going to need to enhance it to add _vv in the near future.

[adamcfraser]

LGTM - ready to merge once dependencies are merged and go.mod updated.