Currently, for simplicity, FilterNodes that express a boolean expression use multiplication to compute selectivity (see 
EvalFilterSelectivityTransport::conjunctionSel, EvalFilterSelectivityTransport::disjunctionSel).
This task should apply exponential backoff to Boolean expressions. One possible simplification is to apply that only to each pair of conjuncts - then it won't be needed to record the estimates of all conjuncts/disjuncts at each level of the expression nesting.