[ENHANCEMENT] Move init function to private

[frank-zsy]

Now, users need to call init by their own and could not make any query before init. Not put init into constructor just because TypeScript not support async constructor.

But actually we can call init in constructor and check inited flag in query so that query will be the only public API that can be used, and since query is async function already, it can use waitFor function to check the inited flag.