Rewrite sgcollect in go

[markspolakovs]

Rewrite sgcollect in go.

• Will almost certainly break the build, as it'll expect Python sgcollect to be present.
• The sgcollect code intentionally doesn't import modules from the existing SG packages (eg. utilities in base or type definitions from rest). This is to keep the binary size down, as all those packages' dependencies will be imported even if they're not used.

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