fix: wait for all packets to be sent before closing the WebSocket connection

This reverts commit [1], which was included in `engine.io@5.1.0` and
`socket.io@4.1.0`.

The WebSocket connection was closed before all packets were written
out, so for example when calling `socket.disconnect(true)` on the
Socket.IO server (which disconnect from all namespaces and close the
connection), the client would receive only the first disconnect packet
and kept trying to reconnect to the other namespaces.

The only difference with the previous implementation (pre 5.1.0) is
that the "drain" event gets only called once at the end, and not after
each packet.

[1]: https://github.com/socketio/engine.io/commit/ad5306aeaedf06ac7a49f791e1b76e55c35a564e

Related: https://github.com/socketio/engine.io/issues/648