Centralize configuration parameters in a Key/Value store

[pjcuadra]

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[pjcuadra]

Even if this isn't the right change at the moment, I'd like to push this a little forward to a point where it's consistent and merge-ready.

I'd just like mention that with this we invert the break the dependency loop between main (venus.cc) and the rest of the program. Having venus.cc depending on the rest of the program but not vice-versa.

This improves testability and decoupling substantially. For instance, I was able to test CacheFile completely independent from everything else.