Clean up abstract ChannelHandler impls / Remove ChannelHandlerContext.hasNext*()

- Rename ChannelHandlerAdapter to ChannelDuplexHandler
- Add ChannelHandlerAdapter that implements only ChannelHandler
- Rename CombinedChannelHandler to CombinedChannelDuplexHandler and
  improve runtime validation
- Remove ChannelInbound/OutboundHandlerAdapter which are not useful
- Make ChannelOutboundByteHandlerAdapter similar to
  ChannelInboundByteHandlerAdapter
- Make the tail and head handler of DefaultChannelPipeline accept both
  bytes and messages.  ChannelHandlerContext.hasNext*() were removed
  because they always return true now.
- Removed various unnecessary null checks.
- Correct method/field names:
  inboundBufferSuspended -> channelReadSuspended