XBR: add ABJ to cross-language example

[oberstet]

ABJ now supports both the XBR seller and buyer roles

• XBR simple buyer #438
• XBR simple seller #437

There also is an example here https://github.com/crossbario/autobahn-java/tree/master/demo-gallery/src/main/java/io/crossbar/autobahn/demogallery/xbr

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What we would like to demonstrate (have an example for) is the ability of XBR/WAMP to transparently cross language boundaries, and have XBR buyers and sellers happily exchange data between them.

To that end, we already have this:

• https://github.com/crossbario/crossbar-examples/tree/master/xbr/teststack1

which includes ABPy and ABJS.

It would be great to have an ABJ/Netty component there as well, in a new folder java, which replicates the functionality of seller.py and buyer.py in https://github.com/crossbario/crossbar-examples/tree/master/xbr/teststack1/python

the whole example is written so that (in addition to a cfx edge node) all components can run concurrently (that is ABPy Buyer, ABPy Seller, ABJS Buyer, ABJS Seller, and once we have this issue resolved, also ABJ Buyer and ABJ Seller)