Internal: Inverse DocIdSets' heuristic to find out fast DocIdSets.

DocIdSets.isFast(DocIdSet) has two issues:
 - it works on the DocIdSet interface while some doc sets can generate either
   slow or fast sets depending on their options (eg. whether an OrDocIdSet is
   fast or not depends on the wrapped clauses).
 - it only works because the result of this method is only taken into account
   when a DocIdSet has non-null `bits()`.

This commit changes this method to work on top of a DocIdSetIterator and to use
a black-list rather than a white list: slow iterators should really be the
exception rather than the rule.

Close #8380