Simple support for instrumenting incoming WebSockets

Today incoming WebSocket connections are not instrumented as we don't know how to group events on the socket into meaningful Transactions.

Normally we refer users to manually instrument the WebSockets using [custom transactions](https://www.elastic.co/guide/en/apm/agent/nodejs/0.x/custom-transactions.html), but it would be really nice if we could provide a better out-of-the-box experience.

If it makes the task easier, we could consider limiting this support to [Socket.IO](https://socket.io/), as that library have higher level concepts that translate better to Transactions.