Separate public APIs from private ones and update API documentation generation logic

[puzpuzpuz]

Currently we don't have a clean separation between public and private APIs of the client. What's even worse, generated documentation includes all private classes, like this one: http://hazelcast.github.io/hazelcast-nodejs-client/api/3.12.3/docs/modules/_util_.html

We should extract interfaces where necessary (as an example, Client should be a TS interface) and make sure that we generate documentation only for such public APIs.