[#1236] Fix problem where adding a new ChannelHandler could block the eventloop

This change also introduce a few other changes which was needed:
 * ChannelHandler.beforeAdd(...) and ChannelHandler.beforeRemove(...) were removed
 * ChannelHandler.afterAdd(...) -> handlerAdded(...)
 * ChannelHandler.afterRemoved(...) -> handlerRemoved(...)
 * SslHandler.handshake() -> SslHandler.hanshakeFuture() as the handshake is triggered automatically after
   the Channel becomes active