Login v2 API

Introducing the v2 Login API, backed by the original Admin.Login logic. Builds on proposed pull request #367.
Did you actually test against old servers with the 'loginV2' call?
Because AFAICT, old servers just ignore the "version=1" part of the request, which means they will login just fine and not complain.
Perhaps 1.20 will complain because it knows about version numbers. I'm pretty sure 1.18 won't complain, which would cause the new client to think it was talking to a new server even though it was actually talking to a much older server.