Netty: Refactoring to make MessageChannelHandler extensible

Small refactorings to make the MessageChannelHandler more extensible.
Also allowed access to the different netty pipelines

This is the fix after the first version had problems with the HTTP
transport due to wrong reusing channel handlers, which is the reason
why tests failed. This version now uses its own ChannelPipelineFactory
again.

Relates #6889 (which had been reverted in the first place)