fix: properly call the send callback during upgrade

The "drain" event (added in [1]) had two different meanings:

- the transport is ready to be written
- the packets are sent over the wire

For the WebSocket and the WebTransport transports, those two events
happen at the same time, but this is not the case for the HTTP
long-polling transport:

- the transport is ready to be written when the client sends a GET request
- the packets are sent over the wire when the server responds to the GET request

Which caused an issue with send callbacks during an upgrade, since the
packets were written but the client would not open a new GET request.

There are now two distinct events: "ready" and "drain"

Related: https://github.com/socketio/engine.io/issues/695

[1]: https://github.com/socketio/engine.io/commit/2a93f06e276f3adb76e494ae9198da38799d493c