Convert all Go code to use Go modules

We would like to convert all Go projects to use Go modules in the Cheshire-Cat timeframe, as it is now the de facto way to manage Go dependencies. It also allows us to make our third-party dependency scans more correct and complete. It also should allow easing of developer management of what versions of third-party code they use.

Full details and instructions are here: https://hub.internal.couchbase.com/confluence/display/CR/Handling+of+Go+Modules+in+Couchbase+Server+Builds