title: how to control maximum number of established connections
body: <p>when using netty to programe,
   I occur an question : "control maximum number of established connections"
  Is there such a method can implements this featrue?</p>
  
  <p>just like : "serverBootstrap.option( ChannelOption.SO_BACKLOG,100);"
  which is used to set the maximum muber of  un-accepted queue connections.</p>