Remove MessageList from public API and change ChannelInbound/OutboundHandler accordingly

I must admit MesageList was pain in the ass.  Instead of forcing a
handler always loop over the list of messages, this commit splits
messageReceived(ctx, list) into two event handlers:

- messageReceived(ctx, msg)
- mmessageReceivedLast(ctx)

When Netty reads one or more messages, messageReceived(ctx, msg) event
is triggered for each message.  Once the current read operation is
finished, messageReceivedLast() is triggered to tell the handler that
the last messageReceived() was the last message in the current batch.

Similarly, for outbound, write(ctx, list) has been split into two:

- write(ctx, msg)
- flush(ctx, promise)

Instead of writing a list of message with a promise, a user is now
supposed to call write(msg) multiple times and then call flush() to
actually flush the buffered messages.

Please note that write() doesn't have a promise with it.  You must call
flush() to get notified on completion. (or you can use writeAndFlush())

Other changes:

- Because MessageList is completely hidden, codec framework uses
  List<Object> instead of MessageList as an output parameter.
[#1534] Fix handling of partial frames in SslHandler

* Let SslHandler not extend ByteToMessageDecoder
Remove channelReadSuspended / Rename messageReceived(Last) to channelRead(Complete)

- Remove channelReadSuspended because it's actually same with messageReceivedLast
- Rename messageReceived to channelRead
- Rename messageReceivedLast to channelReadComplete

We renamed messageReceivedLast to channelReadComplete because it
reflects what it really is for.  Also, we renamed messageReceived to
channelRead for consistency in method names.
Merge remote-tracking branch upstream/$:{upbranch}