Support one-step-processing

[bitzl]

For most applications, the usual read/transform/write cycle is suitable (aka "ETL"), with sometimes leaving out a transforming or writing step. However, there are applications that merely convert messages or act as gateways and therefore harldy fit in that framework.

Introduce a Flow API that allows for a single processing step:

<M extends Message> Collection<Message> process(M m)

or

<M extends Message> Collection<Message> translate(M m)