this.join() in _onconnect() and this.leaveAll() in _cleanup should be asynchronized

[zackliu]

Is your feature request related to a problem? Please describe. Some codes in socket.io doesn't await on Promise<void>. E.g. this.join() in _onconnect() and this.leaveAll() in _cleanup Although the built-in adapter can finish them synchronizedly, but not all the customized adapters can finish them sync. Especially some method in adapter's interface can return Promise<void>

Describe the solution you'd like A clear and concise description of what you want to happen.

Codes that invoke async method should await to guarantee order. E.g.

• _onconnect(): https://github.com/socketio/socket.io/blob/main/lib/socket.ts#L603
• _cleanup(): https://github.com/socketio/socket.io/blob/main/lib/socket.ts#L760

[NextThread]

Hey, can I work on this