Slim Query/Table Table: manage variables assignment in VariableStore (Version 2)

This is idential to pull request 525 from Antoine
Only one line is different compared to the code from Antoine. (if I didn't missed anything). But this implementation  needs no change in the protocol messages and therfore less code. Just one additional library functions must be availbale in the slim clients.

I did this pull request quickly to demonstrate the way how to implement. The new suite tests from Antoine pass. I did no further testing.