Add mutually excluded rules in rule-flow

[aitrusgit]

We need "something" to achieve such functionality. Now users had to make their condition closures mutually exclusive to achieve such effect but that leads sometimes to a code overhead and a small evaluation overhead.

Drools has the concept of "activiation-group" (or smtg like it) which basically restricts activation of only one rule per group. However something like this might be an overkill, we can simply set a flag at rules-flow level that modifies such behavior.

I am happy with not supporting that on simple ClosureReducers ( closures expresed inside an array [] and not in a rule flow)