What is a `handler` versus a `callback`?

You use `callback` to handle writes.  Can we use something like the old system, i.e:

Sockets expose:

`_handle_read` and `_handle_write`

Event loop exposes:

```
def register(socket, read_or_write):
  if not socket.fd in self._sockets:
   self._sockets[fd] = socket
  
  with self._lock:
    self._to_modify.append((socket, read_or_write))
```

In the event loop, you do:

```
while 1:
  poll()
  ...
  for fd, events in socks...
    if event & ..:
     self.sockets[fd].handle_read()
    if event & ..:
     self.sockets[fd].handle_write()

   for socket in self._to_modify:
     poller.do_something
```

(This is similar to the old zeromq.pyx code).