Introduce `send` callback about engine.io

[rauchg]

Writing an explanation here since Afshin asked of what we need to do to make this happen.

Basically when a Socket is holding a buffer, we want to tell the transport to write that buffer. Meanwhile, while the transport is writing it (for example, polling might have to wait for an incoming poll GET request), this buffer could grow at the socket level.

The socket will fire this callback when the drain event is executed by the transport, which means the message was sent successfully. The extra complication is that while we're waiting on this event two other events can be emitted by the transport that should clean up this event: error or close. For example, the client might not re-open a GET request, triggering a timeout and a close. Or the write could fail and an error is emitted followed by a close (see websocket write callback).

The other extra complication is that we need to keep track of callbacks even when the transport is not writable yet (when things just get buffered at the socket).

FInally, it'd be cool to emit an event when the socket is closed and things didn't get flushed out (basically when a writeBuffer has items).

[afshinm]

Thanks @guille, I'll start implementing this one right now.

[rauchg]

Also important is to consider

#59

Basically instead of always assuming that things are going to be handled well by the transport and resetting the writeBuffer, we should keep track of the length of the writeBuffer in the closure that's being passed down to the transport, then slice the writeBuffer it it succeeds (because it could keep growing after we commit that buffer to the transport)