Remove special handling of Object[] in codec framework (a.k.a unfolding)

- Fixes #1229
- Primarily written by @normanmaurer and revised by @trustin

This commit removes the notion of unfolding from the codec framework
completely.  Unfolding was introduced in Netty 3.x to work around the
shortcoming of the codec framework where encode() and decode() did not
allow generating multiple messages.

Such a shortcoming can be fixed by changing the signature of encode()
and decode() instead of introducing an obscure workaround like
unfolding.  Therefore, we changed the signature of them in 4.0.

The change is simple, but backward-incompatible.  encode() and decode()
do not return anything.  Instead, the codec framework will pass a
MessageBuf<Object> so encode() and decode() can add the generated
messages into the MessageBuf.