[INTERNAL][CORE] Prepares Stream access

[srossbach]

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This patch introduces an new class / interface which is just used for the ITransmitter and IReceiver. This is necessary to distinguish between plan stream connections and internal stuff.

[srossbach]

@tobous can you just please have a look at the Javadoc and basically just the naming of the methods in the interfaces ? I guess the rest of this "commit" is not reviewable, however the whole logic of the DTM is tested with a bunch of test cases.

[tobous]

@srossbach Yes, I can have a look at it. But I'm busy for the rest of the week, so I won't get to it (or at least finish it) before next Monday. Either way, with this mainly dealing with the network stuff, @stefaus should (also) have a look at this as it might interfere/intersect with his work.

[srossbach]

He has not replied since 9 days, so I removed him as reviewer.

[srossbach]

I currently have limited time. Will reply on friday, saturday, or sunday.