Warming up to the remote collections support

Huge refactoring to have the code in shape to later support the
remote collections with different backends.

src/variables/highest_severity.h

@@ -34,7 +34,7 @@ class HighestSeverity : public Variable {
         : Variable(_name) { }
 
     void evaluateInternal(Transaction *transaction,
-        std::vector<const transaction::Variable *> *l) override;
+        std::vector<const collection::Variable *> *l) override;
 };