Esper Decoupling

[riccardotommasini]

Reduce the coupling with Esper my means of RSP-QL dedicated interfaces.

I can notice that the streams package is quite bounded to the DSMS and should be hidden.
I am considering to delegate the usage of esper in a specific context (RSP engine runtime).
On the other hand, I would like to reduce the amount of code that someone who want to extend yasper must write.