title: Channel.isWritable notofication in Netty
body: <p>I have a scenario where I need to keep writing to a <code>Channel</code>. In case it is unable to take further data  ( because of the buffers being full etc ) , I need to stop writing , but need to resume as soon as the channel becomes writable again. How do I determine this instant when a channel becomes writable again ? Is there a callback event that is fired which I can override ? I am extending <code>SimpleChannelUpstreamHandler</code> in my business handler - this has a method <code>channelInterestChanged()</code> - but not sure if this is correct hook to tap into ?</p>
  
  <p>I was hoping to avoid using primitive thread-based wait mechanisms like wait-notify as these will involve context switching. Any non-blocking way to achieve this wait ?</p>