Adapter method should return a Promise | void

[zackliu]

Is your feature request related to a problem? Please describe. Some Adapter method return void only that make customized adapter implementer have trouble on implementing. E.g. io.socketsJoin() and inner adapter's method addSockets and delSockets currently return void which brings troubles for new adapter implementer as the adapter may not be able to do it synchronized.

Describe the solution you'd like Return Promise | void instead of void and thus let adapter implementer decide whether it should be async.

[NextThread]

Can I work on this?

[dhardtke]

This is probably related to https://github.com/socketio/socket.io/issues/4734 and I'd love to see it fixed.