Add ChannelInboundConsumingHandler

..which is useful when the handler is placed at the last position of the
pipeline because it releases the received messages automatically.
Merge ChannelInboundConsumingHandler into SimpleChannelInboundHandler

- SimpleChannelInboundHandler now has a constructor parameter to let a
  user decide to enable automatic message release. (the default is to
  enable), which makes ChannelInboundConsumingHandler of less value.