CBG-4459 validate cv when passed in

[torcolvin]

CBG-4459 validate cv when passed in

• validate that sourceID is base64, requires some test changes
• pass back better error messages if hlv was not found

The sourceID from CBL/CBS never include padding, so I used base64.RawStdEncoding to generate sourceIDs. Not all ASCII strings are valid base64, so I changed some strings.

I considered doing regexp parsing for invalid characters, since what was happening before this PR was that "1@abc; 2@def" was getting parsed as SourceID=abc; 2@def Value=1. I decided to just parse as base64 since this will provide more robust validation. There are two possible downsides to this: it is slightly more expensive to call ParseVersion and we can't use any artibrary string for source id for debugging.

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/2899/