Improve Codec API

[mp911de]

The Codec API is based on RedisCodec which defines methods to encode and decode keys and values. The class has following issues:

• asymmetric methods: encode returns byte[] whereas decode accepts ByteBuffer
• The class is abstract, but could be easily an interface (raised by @prepor)
• Users have to create a byte[]. This is not explicitly necessary. All encoded values are written into a nio ByteBuffer, so only a source for bytes is needed (raised by @beckje01)

Constraints of the improved API:

• Break as little as possible and preserve RedisCodec as much as possible
• Change the connect() methods in a compatible way

Proposed change:

1. Introduce a new abstraction of RedisCodec that returns ByteBuffer on encode Another abstraction requires to change all API's where RedisCodec is referenced. Plan B: Turn RedisCodec into an interface
2. Adopt existing codecs
3. Explain migration in the 3.x to 4.x migration guide

Why ByteBuffer? ByteBuffers are handles to memory and encapsulate data. Netty's ByteBufs work internally with ByteBuffers and can produce references to the data without actually duplicating it. ByteBuffer is used already in RedisCodec.

[mp911de]

After implementing the code, I'm not sure whether it's worth the change.

[beckje01]

I think it looks good and eliminates the byte duplication that was required with the byte[] at the codec stage. Also all for switching it to an interface, looks nice and clean.

[mp911de]

Implemented for 4.0