getAllKeyRange = function() {
    return new lf.index.SingleKeyRangeSet([lf.index.SingleKeyRange.all()]);
  }
Traverses the indexed columns in reverse order and fills in an "all"
SingleKeyRangeSet where possible in the provided map.
Example1: Assume that the indexed columns are ['A', 'B', 'C'] and A is
already bound, but B and C are unbound. Key ranges for B and C will be filled
in with an "all" key range.
Example2: Assume that the indexed columns are ['A', 'B', 'C', 'D'] and A, C
are already bound, but B and D are unbound. Key ranges only for D will be
filled in. In practice such a case will have already been rejected by
IndexRangeCandidate#isUsable and should never occur here.
@param {!lf.structs.Map<string, !lf.index.SingleKeyRangeSet>} keyRangeMap
@private