Split HttpPostRequestDecoder into HttpPostStandardRequestDecoder and HttpPostMultipartRequestDecoder / Add HttpData.maxSize

- Related issues: #1937 #1938 and #1946
- Add InterfaceHttpPostRequestDecoder and Make HttpPostRequestDecoder
  implement it
- HttpPostRequestDecoder actually delegates itself to
  HttpPostStandardRequestDecoder or HttpPostMultipartRequestDecoder
- Deprecate IncompatibleDataDecoderException because it's not thrown
  anywhere now
Fix some inspector warnings